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Why are men more interested than women in casual sex and sexual variety?

The standard answer is simple: Men can father more children, so they evolved to want more partners.

It’s a nice, tidy story - and it’s partly true.

But it also fails to explain a lot of what we actually see in the world. In many species, females are far more promiscuous than the standard story predicts. And in our own species, sex differences in sexual psychology, though real, are smaller than you might expect.

In this excerpt from my forthcoming book A Billion Years of Sex Differences, I lay out a new way of thinking about the evolution of sex differences in sexual psychology - one that goes beyond the usual focus on maximum offspring number and brings a wider range of evolutionary forces into view.

You can access the full collection of excerpts here, and preorder the book here (UK) or here (US).

The Evolution of Sex Differences in Sexual Psychology: It’s Not (Just) What You Think

We saw in the last excerpt that sex differences in sexual psychology aren’t just a product of culture; there’s also an innate contribution. Importantly, though, to say that there’s an innate contribution is to say nothing about how it came about. Why did natural selection ‘decide’ to give men and women somewhat different, somewhat incompatible desires? If a god were behind it, we might suspect that he was playing a cruel joke on us or trying to make the soap opera of human life more entertaining for himself. With natural selection as the architect, however, no such explanation is available. Somehow, male-typical sexual leanings must have been adaptive for our male ancestors, whereas female-typical sexual leanings must have been adaptive for our female ones. The only real question is why.

Now, you might think you know the answer already. I touched on it in Chapter 4, and in any case, the basics are quite well understood these days. But not so fast. As with sexual selection, the evolutionary explanation for sex differences in sexual inclinations is long overdue for an overhaul. The aim of this section is to sketch out a new and improved version of the explanation. We’ll start with the standard story, then gradually thicken the plot.

The standard story goes like this. As a result of sex differences in reproductive physiology, men can potentially have many more offspring than women. A man who has, say, five sexual partners in a year could end up with five new offspring; in contrast, a woman who has five sexual partners in a year would probably end up with no more offspring than she would if she’d just had one.

This created very different selection pressures for our male vs our female ancestors. Men who sought multiple partners typically had more offspring than men with less wide-ranging interests, and the genes underpinning that tendency became increasingly common in the human gene pool. Men evolved a higher sex drive, a greater hunger for casual sex, a stronger desire for sexual variety, and a capacity to be quickly aroused by sexual stimuli (which for most of our evolution meant flesh-and-blood women rather than porn).

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Long before they can walk, talk, or tie their shoelaces, infants have a rudimentary sense of beauty. By four months of age, babies look longer at patterns that adults judge to be beautiful. This suggests that, like our love of faces, the aesthetic sense may be built into the human mind from the start.

The graph comes from a recent paper by Hélène Mottier and colleagues. Here’s the abstract:

The present study investigated the emergence of a visual preference for beautiful kinetic dot displays during development. The displays were previously judged for relative beauty by an independent group of adults. Preferential looking was measured with an eye tracker in 4- to 24-month-old infants and adults. Analysis of the overall preferential looking response over the 5 s of stimulus display indicated that adults’ judgement predicted preferential looking at all ages tested. Analysis of the time course of this attentional response indicated two different mechanisms: (i) a fast-orienting response towards motion patterns that were not judged as beautiful by adults, and (ii) a slower but longer-duration response towards motion patterns that were judged as beautiful by adults. The contribution of these two mechanisms to the overall preferential looking response changed with age in a consistent manner. Because the beauty ratings of adults were associated with a later but longer-duration visual attention response in infants, and most preferred patterns differed in many aspects from non-preferred patterns, we propose that a sense of beauty, defined in adults as a pleasurable mental state leading to sustained visual attention, may influence behaviour by four months of age when looking at kinetic dot displays.

You can access the paper here for free.

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How You Can Support the Newsletter

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Related Reading From the Archive

In Case You Missed It…

• Top 10 Most Replicable Findings from Behavior Genetics, Part 2: Findings #6-10

• Keeping It Casual, Part 2: The Nature and Nurture of Sex Differences in Casual Sex

• People Prone to Corporate Bullshit Tend to Make Worse Leaders

Welcome to the April 2026 edition of the N³ Newsletter Linkfest: a collection of links to papers and articles that grabbed my attention over the last month. On the menu today…

• An innate sense of kindness in humans and dogs

• Why greater gender equality means fewer babies

• Three new findings on personality, three new findings on sex differences, and three new findings on gender issues

• The lengths females go to to avoid unwanted male attention

• The downside of reclaiming slurs

• And we’ll ask: Which is more common, misogyny or misandry? Does gender reassignment help or harm the mental health of youth with gender dysphoria? And does DEI foster inclusion and mutual understanding or division and hostility?

The first half of the Linkfest is free to read for everyone; the rest is for paid subscribers. I tend to put my spicier content behind a paywall, so if you want a taste of that, consider becoming a paid subscriber.

You can read about the benefits of a paid subscription here, and you can access the complete collection of Linkfests here.

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But First, Some News…

I’ll be appearing at the How the Light Gets In festival in Hay-on-Wye this May 22-25, doing a debate about the merits and demerits of spirituality, a podcast with , and an event for my forthcoming book A Billion Years of Sex Differences.

Wanna come? You should come; it’ll be fun!

Explore tickets

Echolocation in Humans

Some blind people don’t just learn to live without vision; they replace it with something arguably just as impressive: echolocation. Specifically, they make click sounds with their mouths, then use the returning echoes to detect objects and navigate through the world. In a clever EEG study, blind echolocators outperformed sighted participants at locating objects in the dark, progressively refining their spatial judgments by integrating echoes across multiple clicks. [Link.]

Born to Give

Toddlers, it turns out, are tiny altruists: They’re happier giving treats away than receiving them. Not only that, but they’re happier when they personally give the treats than when someone else does, and happier when they give them spontaneously than because they were told to. This is suggestive evidence that human altruism is driven by the inbuilt emotional rewards of lending a helping hand. [Link.]

Dogs Help, Cats Watch

Dogs are about as helpful as toddlers, but cats look out for number one. In a fascinating recent experiment, a human parent pretended to be looking for a lost object. Around three-quarters of toddlers and dogs tried to help. Most cats, in contrast, sat back unless there was something in it for them. Helpfulness, it seems, comes more naturally to some species than others. [Link.]

More Equality, Fewer Babies

Fact: Across countries, greater gender equality is associated with lower fertility. Theory: With greater freedom and material wealth, women become less dependent on men, and thus less likely to form pair bonds. Combine this with reliable contraception, and the fertility rate comes crashing down. [Link.]

The Missing 5’11” Men

Something strange is happening with self-reported height among men: There aren’t enough 5’11” men, and there are too many 6’0” men. Hmmm... [Link.]

The Paradox of Upward Mobility

We tend to assume that people who are born poor and get rich will be more sympathetic to the plight of the poor than those who are born rich and stay rich. But it’s the other way around: Those who start out poor are less sympathetic. [Link.]

A Billion Years of Plate Tectonics

This incredible animation compresses a billion years of plate tectonics into just 40 seconds. We tend to think of land masses as stable - but they’re in constant motion. They just move a lot more slowly than us. [Link.]

Three New Findings on Personality

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In Case You Missed It…

• Top 10 Most Replicable Findings from Behavior Genetics, Part 1: Findings #1-5

• The Second Law of Behavior Genetics

• 10 Highly Replicable Findings from Psychology

What if upbringing matters far less than we think, and the environment is shaped in part by our genes? What if many familiar “environmental” effects aren’t really environmental at all?

These ideas might strike you as counterintuitive or even obviously wrong. But they’re not just random hypotheticals. They’re among the most robust and consistently replicated findings in behavior genetics - a field that’s produced some of the strongest results in all of psychology.

In this post, I finish outlining the Top 10 most replicable findings from behavior genetics, first identified in a classic paper by Robert Plomin and colleagues.

And I ask: Why have these findings proved so unusually robust when so much of psychology has failed to replicate? What’s the secret sauce?

If you’re interested in what bedrock looks like in psychological science - and in ideas that might quietly reshape how you think about human nature - this post is for you.

6. Psychological Stability Is Largely Genetic

Why do people remain recognizably themselves over the years? Why do smart people tend to stay smart, shy people tend to stay shy, and depression-prone people tend to stay depression-prone?

Behavior genetics provides a clear answer: Psychological stability is largely due to genes. The same genetic influences tend to operate across the lifespan, helping anchor individual differences over time.

Psychological change, in contrast, is more often environmental in origin. New experiences, new life events, and shifting contexts nudge around our psychological dispositions in ways not directly predictable from our genes.

The fact that genes explain psychological stability rather than change might sound obvious, but it’s not. Genes don’t just switch on at conception, then remain active till the last days of life. Many remain dormant until needed at puberty or other key life junctures. In principle, then, genes could help explain psychological change as well as stability. As it happens, though, they generally don’t. For the most part, stability is genetic, whereas change is a function of the environment.

7. The Environment is Partially Heritable

Now things get really interesting.

Not only are all psychological traits heritable, but the environment is heritable too. Even variables we think of as entirely environmental - parental treatment, social support, major life events - are shaped in part by our genes.

How can this be?

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Across cultures and centuries, men have shown a stronger interest in casual sex and sexual variety than women. Or so says common wisdom. But is common wisdom right?

In the first post in this series, I argued that it is. In this post, I’ll tackle the harder question: Where do the differences come from? Are they purely products of social pressure, double standards, and patriarchy - or do they run deeper than that?

Long story short, the evidence points strongly toward the latter. Among other things, sex differences in sexual psychology are stubbornly persistent over time, consistent across cultures, and - most tellingly - mirrored in other species, from baboons to hedgehogs to fruit flies. That’s hard to blame on the patriarchy.

If you want to understand what really drives one of the largest and most debated differences in human psychology, read on.

This post is an excerpt from my forthcoming book A Billion Years of Sex Differences. You can access the full collection of excerpts here, and preorder the book here (UK) or here (US).

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People who fall for buzzword-laden corporate waffle tend to be worse leaders and poorer decision-makers. Receptivity to “corporate bullshit” is linked to lower analytic thinking (first graph below) and worse judgment in workplace contexts (second graph).

1. Corporate Bullshit and Analytical Thinking

2. Corporate Bullshit and Workplace Judgement

The graphs come from a fascinating recent paper by Shane Littrell. Here’s the abstract:

From boardrooms and brown bags to emails and earnings calls, business culture often seems overrun by “corporate bullshit,” a semantically empty and often confusing style of communication in organizational contexts that leverages abstruse corporate buzzwords and jargon in a functionally misleading way. At best, corporate bullshit can sometimes seem harmless. At worst, it can disrupt organizational and employee effectiveness in numerous ways including obstructing clear communication, increasing employee disengagement, tarnishing a company’s reputation, and exposing businesses to potential financial and legal risks. Here, results from four studies (total N = 1018) report the construction and validation of the Corporate Bullshit Receptivity Scale (CBSR), a novel measure of individual differences in susceptibility to corporate bullshit. Results show that corporate bullshit receptivity is distinct from a general affinity for corporate speech, negatively associated with measures of analytic thinking, and positively related with other bullshit-related constructs in theoretically-consistent ways. Importantly, corporate bullshit receptivity is positively associated with several workplace perception variables and is a robust negative predictor of work-related decision-making. Overall, the findings establish the CBSR as a valid and reliable tool to aid researchers and practitioners in examining the causes, correlates, and consequences of receptivity to bullshit in organizations.

You can access the paper here or download a free version here.

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How You Can Support the Newsletter

This post was free to read for all. If you like what I’m doing with The Nature-Nurture-Nietzsche Newsletter, and want to support my work, there are several ways you can do it.

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In Case You Missed It…

• Keeping It Casual, Part 1: Sex Differences in Interest in Casual Sex

• 10 Highly Replicable Findings from Psychology

• The First Law of Behavior Genetics

Psychology has been having something of an identity crisis lately.

Over the past few decades, a growing number of classic findings have failed to replicate. Indeed, one of the most replicable findings in psychology, ironically enough, is that only around 50% of psychology findings are replicable.

Psychology isn’t unique in this respect; similar concerns have been raised in many other fields. But psychology is a particularly popular field, and one with clear implications for policy, so its potential to mislead is great.

That being the case, it’s important to ask: Which findings in psychology do replicate?

One area with a particularly good track record is behavior genetics, which studies the nature and nurture of individual differences in mind and behavior. In a classic paper drawing on decades of twin, adoption, and genomic research, Robert Plomin and colleagues highlighted ten findings from the field that have proven remarkably robust. These aren’t trivial or esoteric findings. They’re large and often surprising, and have far‑reaching implications for our understanding of psychology.

In this post, I’ll outline the first five of Plomin and co.’s Top 10 most replicable findings from behavior genetics, covering everything from selectively breeding the minds of other animals to the increasing importance of genes with age. I’ll tackle findings #6-10 next weekend.

1. All Psychological Traits Show Genetic Influence

The first finding is sometimes known as the First Law of Behavior Genetics: Individual differences in all psychological traits are shaped in part by individual differences in genes.

This is true of intelligence, personality, mental illness, substance use, political attitudes, religiosity - even food preferences and social media habits. Across thousands of studies, identical twins are consistently more similar than fraternal twins on these traits, and adopted children are more similar to their biological parents than their adoptive ones. Twin and adoption studies have very different underlying logics, yet they converge on the same conclusion: Genes matter.

And often they matter a lot. For intelligence, heritability averages around 50% across diverse countries and cultures. For personality, it usually falls between 30% and 50%. For mental-health problems such as schizophrenia and autism, it clocks in at 80% or more: on a par with the heritability of height.

To be clear, heritability isn’t static; it varies from place to place and time to time. But we’ve yet to find a place or time where it’s negligible for any major psychological trait.

The First Law of Behavior Genetics wasn’t always widely accepted. Indeed, in some fields, such as education, it’s still not. However, within psychology and allied disciplines, it’s now recognized as one of the best-established findings we have.

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2. No Psychological Traits Are 100% Heritable

The first finding emphasizes genes; the second restores balance to the force. Genes matter a lot, but so do non-genetic influences. No psychological trait is 100% heritable, and most are nowhere near it.

This isn’t due to limitations in the methods of behavior genetics. Some physical traits have extremely high heritability levels - up to 90% for height, for example. Thus, the methods could give higher heritability estimates if heritability were actually higher.

Admittedly, psychological measures are usually noisier than physical ones, which depresses heritability estimates to some degree. Even so, the estimates are far enough away from 100% that improved measurement is unlikely to bring them anywhere close to that ceiling.

This finding matters more than it might first appear. It’s natural to assume that behavior genetics tells us about genes rather than the environment. In fact, though, it tells us about both. More than that, behavior genetics provides the strongest evidence we have that the environment plays a crucial role in shaping individual differences. That’s because the methods of behavior genetics control for the effects of genes, whereas other methods don’t. Given how substantial genetic effects are, the failure of most socialization research to do this is a profound oversight. Ironically, then, the clearest demonstration of the importance of the environment comes from a field often assumed to downplay it.

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A new Nature paper by Ali Akbari, David Reich, and colleagues explores recent evolution - that is, evolution in the last 14,000 years - in West Eurasians.

Comparing the genomes of ancient West Eurasians to those of modern ones, they found strong evidence that gene variants associated with intelligence were under positive selection, whereas gene variants associated with body fat, darker skin, and disorders like schizophrenia and bipolar were under negative selection.

The body-fat finding is particularly interesting; it fits with the idea that farming reduced the need to stockpile fat on the body, and thus that farming populations evolved to do less of it.

Here’s the abstract:

Ancient DNA has transformed our understanding of population history, but its potential to reveal as much about human evolutionary biology has not been realized because of limited sample sizes and the difficulty of distinguishing sustained rises in allele frequency increasing fitness—directional selection—from shifts due to migrations, population structure, or non-adaptive purifying or stabilizing selection. Here we present a method for detecting directional selection in ancient DNA time-series data that tests for consistent trends in allele frequency change over time, and apply it to 15,836 West Eurasians (10,016 with new data). Previous work has shown that classic hard sweeps driving advantageous mutations to fixation have been rare over the broad span of human evolution. By contrast, in the past ten millennia, we find that many hundreds of alleles have been affected by strong directional selection. We also document one-standard-deviation changes on the scale of modern variation in combinations of alleles that today predict complex traits. This includes decreases in predicted body fat and schizophrenia, and increases in measures of cognitive performance. These effects were measured in industrialized societies, and it remains unclear how these relate to phenotypes that were adaptive in the past. We estimate selection coefficients at 9.7 million variants, enabling study of how Darwinian forces couple to allelic effects and shape the genetic architecture of complex traits.

You can access the paper here for free.

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Men and women are similar in many ways - but if there’s one area where we might expect them to differ markedly, it’s sexual behaviour.

In this excerpt from my forthcoming book, A Billion Years of Sex Differences, I kick off a three-part exploration of female-male differences in sexual psychology, focusing in particular on attitudes to casual, no-strings-attached sex.

• In the first instalment, I examine the evidence for these sex differences: Are they real, or just old-fashioned stereotypes?

• In the second, I look at the origins of the differences: Are they purely a product of culture, or is there also an innate contribution?

• And in the third, I consider the evolution of the differences: Why might natural selection have favoured somewhat different sexual inclinations in men than in women?

You can access the full collection of excerpts here, and preorder A Billion Years of Sex Differences here (UK) or here (US).

But First, an Announcement…

I’ll be promoting A Billion Years at the How the Light Gets In festival in Hay-on-Wye this May. If you want to join me for champagne, cake, and a discussion of the evolution of sex differences, click here to book your place!

Keeping It Casual, Part 1

Perhaps you’ve heard the rumours, but men and women aren’t always on the same page when it comes to sex and love. This is nicely illustrated by the story of a young woman who, tired of receiving unsolicited dick pics from every Tom, Richard, and Harry, decided to return fire with her own ‘crotch shot’. To her surprise, the recipients didn’t react the way she had. Rather than being shocked or chastened, they were over the moon.

The behaviour of the woman in this scenario, and that of the men who sent the dick pics to begin with, exemplifies what psychologists call failures of cross-sex mindreading. Why are such mindreading mishaps so common?

After many years of studying and seeking an answer, I’ve hit upon a radical new theory that, in my view, lays this age-old mystery to rest: Men and women are different.

Wait a minute, you might protest; didn’t you just finish arguing that men and women aren’t particularly different? Well, I did. Sex differences in humans are generally modest. However, when it comes to sexual psychology, the gaps are notably larger. In fact, it’s in the realm of sexuality that we find the single largest psychological sex difference ever documented in our species. (See if you can guess what it is; I’ll come back to it later.)

Certainly, most sex differences in sexuality aren’t like night and day. But they are big enough to matter. That’s why the shelves of most bookstores are groaning under the weight of books about navigating sex and relationships. Let’s explore some of the sex differences that turn these tomes into bestsellers.

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In Case You Missed It…

• 12 Things Everyone Should Know About the Psychology of Victimhood

• 12 Things Everyone Should Know About Political Psychology

• The People Most Confident in Their Political Views Know the Least About Politics

Liberals and conservatives both want to reduce harm and help victims. Where they differ is in who they see as the victims.

The disagreement isn’t that liberals think Black people are the victims whereas conservatives think White people are, or that liberals think poor people are the victims whereas conservatives think rich people are.

Instead, each side conceptualizes victimhood in a fundamentally different way.

Liberals tend to see victimhood as group-based, dividing the social world into vulnerable oppressed groups and invulnerable oppressors. Conservatives, in contrast, tend to see victimhood as individual-based and more evenly distributed across groups.

This pattern is shown in the graph below.

The graph comes from a recent paper by Jake Womick and colleagues. Here’s the abstract:

Moral disagreement across politics revolves around the key question, “Who is a victim?” Twelve studies explain moral conflict with assumptions of vulnerability (AoVs): liberals and conservatives disagree about who is especially vulnerable to victimization, harm, and mistreatment. AoVs predict moral judgments, implicit attitudes, and charitable behavior—and explain the link between ideology and moral judgment (usually better than moral foundations). Four clusters of targets—the Environment, the Othered, the Powerful, and the Divine—explain many political debates, from immigration and policing to religion and racism. In general, liberals see vulnerability as group-based, dividing the moral world into groups of vulnerable victims and invulnerable oppressors. Conservatives downplay group-based differences, seeing vulnerability as more individual and evenly distributed. AoVs can be experimentally manipulated and causally impact moral evaluations. These results support a universal harm-based moral mind (Theory of Dyadic Morality): moral disagreement reflects different understandings of harm, not different foundations.

You can access the paper here or download a free version here.

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How You Can Support the Newsletter

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In Case You Missed It…

• Who Engages in More Science Denial, Left or Right?

• Who Really Wants Sex More, Men or Women?

• 12 Things Everyone Should Know About Wokeness

This is my 300th post on Substack, and the second installment of my new “Warnings” series. You can access the full collection here. Hope you enjoy it!

For many of us, politics isn’t just something that happens “out there.” It’s woven into everyday life - and not always to our benefit. Sometimes, in fact, it can be deeply upsetting. Could the constant drumbeat of negative political news be undermining our mental health?

A recent paper in the Journal of Personality and Social Psychology tackles this important question, and concludes that the answer is yes. Day-to-day political exposure reliably evokes negative emotions - anger, sadness, disgust, and the like - which in turn erode both psychological and physical well-being.

How do people deal with the ever-present drip, drip, drip of negative news? How does it affect their motivation to engage in political action? And are these effects stronger on one side of the political aisle than the other?

All will be revealed in this post!

From Big Shocks to Daily Drips

We sometimes think of politics as something distant, played out in parliaments, ministries, and presidential palaces a million miles away. But politics often seeps into day-to-day life. Turn on the news, scroll through social media, sit down at dinner, or chat with colleagues, and there it is. For many of us, politics has become a central feature of our lives and identities. In a very real sense, the political is personal.

How, then, does politics affect people’s well-being? Much of the existing research on this topic has focused on major events such as elections, referenda, and political upheavals. And the findings are, in a way, reassuring. Even when people are deeply invested, the emotional impact of these events tends to fade quickly. For example, the 2016 US presidential election - despite its intensity - didn’t lead to lasting declines in well-being among those disappointed by the result. Most people quickly adjusted to the new normal.

However, politics isn’t just a series of isolated shocks. It’s a continuous stream of events: scandals, conflicts, policy announcements, controversies. Each on its own might be minor. Together, however, they form a background hum of tension. Politics, in other words, may be a chronic stressor - and chronic stressors can undermine psychological and even physical well-being.

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In Case You Missed It…

• Who Engages in More Science Denial, Left or Right?

• Who Really Wants Sex More, Men or Women?

• 12 Things Everyone Should Know About Political Psychology

When it comes to politics, confidence and competence don’t always go hand in hand. In fact, often they go in opposite directions: The people with the least political knowledge are the most confident that they’ve got everything figured out - as shown in the figure below. (FOK = feeling of knowing; CJ = confidence judgments.)

The figure comes from a paper by Erika Fulton and colleagues. Here’s the abstract:

Accurate metacognition is essential to informed political decisions and preventing the spread of misinformation, but research on metacognitive monitoring of political facts, outside of misinformation studies, has not met metacognitive research measurement standards. We addressed this limitation and assessed potential factors (political knowledge, political orientation, and cognitive style) related to this type of metacognitive monitoring accuracy. A sample of 216 Amazon Mechanical Turk participants completed measures of political orientation, political knowledge, and cognitive style, along with a test of political metacognitive monitoring accuracy, between February 2021 and March 2022. Participants were generally overconfident in their political knowledge, especially those with less political knowledge and who were more conservative, with conservatives also showing worse relative metacognitive accuracy. Cognitive style partially explained differences due to political orientation. Our participants were U.S. residents and disproportionately White, male, and lower-to-middle income, so studies of other samples and political contexts are needed for generalizability. Results also build on cue theory, showing that political knowledge and political orientation can act as metacognitive judgment cues. The study highlights the need to improve political metacognitive monitoring accuracy and reveals three targets for intervention: political knowledge, political orientation, and cognitive style.

You can access the paper here or request a free copy here.

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How You Can Support the Newsletter

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3. Upgrade to Paid: A paid subscription gets you:

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Steve

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In Case You Missed It…

• The Evolution of Mutual Mate Choice

• 5 Replication Failures, Questioning DEI, and Sex Differences in Brain and Behavior

• 10 Highly Replicable Findings from Psychology

As a reader of The Nature-Nurture-Nietzsche Newsletter, you’ve no doubt heard of the replication crisis: the finding that only around half of studies in the social sciences replicate. Well, here’s a question for you: Is that finding itself replicable?

A massive new study by Andrew Tyner and colleagues, published in the journal Nature, addresses that thorny question, and comes to a strong conclusion: Yes.

The researchers attempted to replicate 274 findings drawn from 164 papers published between 2009 and 2018, spanning 54 journals across the social and behavioral sciences. The replication studies were well-powered, with a greater than 99% chance of detecting the original effect if the effect were real. The team used the original materials wherever possible, and the replication plans were peer-reviewed in advance. In short, this was about as careful and systematic a replication effort as you could hope for.

What did they find? The headline result is that only around 55% of the original results held up. Not only that, but among those that did replicate, the effect sizes were notably smaller in the replications - roughly half the size of the originals.

Different fields showed somewhat different replication rates. As the graph below shows, education was in first place, with economics bringing up the rear. My own field of psychology fell somewhere in the middle of the pack. That’s not much of a boast, though, given that there’s only about a 50% chance that any given psychological finding will replicate.

On the one hand, this is somewhat depressing news. On the other, we shouldn’t forget that some findings in psychology are as solid as anything else in science. As I argue in the essay linked below, not everything you were taught in intro psych was false!

The Nature-Nurture-Nietzsche Newsletter

10 Highly Replicable Findings from Psychology

Psychology’s Reputation Problem…

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a year ago · 86 likes · 15 comments · Steve Stewart-Williams

• You can access the Tyner paper here.

• You can request a free copy here.

• In a few weeks, I’ll be publishing a piece on the Top 10 most replicable findings from behavior genetics. Stay tuned!

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Most mammals fit a familiar pattern: Males compete for females; females choose from among the males on offer. Humans, at first glance, seem to follow suit. But look a little closer, and a more interesting picture emerges - one in which both sexes compete, both sexes choose, and the usual story about the evolution of human sex differences needs a major overhaul.

In this excerpt from my forthcoming book A Billion Years of Sex Differences, I outline a theory I developed with Andrew Thomas about how the extraordinary demands of raising human offspring helped transform our species from a fairly typical mammal into something closer to a pair-bonding bird: a species characterized by mutual mate choice, in which both sexes have their equivalents of the peacock’s tail and the deer’s antlers.

You can access the full collection of excerpts here, and preorder the book here (UK) or here (US).

The Evolution of Mutual Mate Choice

In many ways, as you’ve no doubt already noticed, our species fits the same mould as most other animals, and particularly other mammals. For a start, the minimum biological contribution required to make a new human being is larger for women than men. Women’s minimum is a nine-month pregnancy and – until recently – several years of breastfeeding. Men’s minimum is notably more modest: in principle, as little as a single sex act. The philosopher and psychologist Cordelia Fine captured the asymmetry like this:

As we all know, the two parents of every human baby are owed grossly unequal debts for the miracle of life. According to my rough calculations, the mother is due more or less a lifetime of unwavering gratitude in return for the donation of a nice plump egg, forty weeks or so full bed and board in utero, many hours of labour, and several months of breast-feeding. But for the father, who by the time of birth may have supplied nothing more than a single sperm, a quick appreciative nod might well seem sufficient.

(Note that Fine isn’t a fan of the evolutionary approach to sex differences, and that the passage above comes from a book critiquing it. But she’s a great writer, and I love the passage!)

As a result of the sex difference in minimum parental investment, men can potentially have many more offspring than women. Of course, the fact that men can potentially do this doesn’t necessarily mean any do. But sociological, anthropological, and genetic evidence indicates that, in virtually every society, men’s reproductive output is more variable than women’s, with more men than women having many offspring and more men than women having none. And just as we’d expect in such a species, humans exhibit most of the sex differences found in our nonhuman kin: Women are often more selective about their sexual partners, men are often more aggressive, and so on – the usual list. At first glance, then, we seem to be typical mammals with typical mammalian sex differences.

But it’s time for a twist in the plot. Humans, you see, turn out to be anything but typical. As I argued in a 2013 paper in Psychological Inquiry, co-authored with my then-PhD student Andrew Thomas, there are two main ways we can construe humans’ evolved sexual psychology. The first we dubbed the males-compete/females-choose (or MCFC) model. According to this model, humans are a highly dimorphic species, in which males compete for mates, and females choose from among the males on offer. The second model we called the mutual-mate-choice (or MMC) model. According to this model, humans are a relatively monomorphic species: a species in which both sexes are choosy about their long-term mates, and both compete among themselves for the most desirable mates in their league. As you’ve probably guessed, we argued that the latter is a better fit for our species.

Certainly, women are often choosier than men, and certainly, men often compete in more extreme and risky ways. However, these are differences of degree rather than kind, and compared to most other mammals, sex differences in our species are modest. Rather than being highly dimorphic critters like gorillas or red deer, humans are somewhat monomorphic ones, like gibbons or grebes. Here’s our five-step explanation for how we came to be this way.

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Twice as many people die of cancer today as did in the 1980s.

At first glance, this sounds like bad news. It seems to suggest we’re losing the war on cancer.

But not so fast.

The raw number of cancer deaths has risen - but so has the size of the population. Once we take that into account, the picture changes. Adjusted for population growth, cancer deaths per capita are roughly the same as they were in the 80s.

This might suggest we’ve made no real progress in the war on cancer - that we’ve just been treading water.

But again, not so fast.

Cancer deaths per capita haven’t budged, but the population is older today than it was in the 1980s, and cancer is more common among older people. A fair comparison would take age into account as well. When we do that - using what epidemiologists call age-standardized rates - we find we’ve actually made considerable progress in the fight against cancer.

Specifically, someone who gets cancer today is much more likely to survive than someone of the same age who got cancer forty years ago - as shown in the graph below.

This is a good example of how easy it is to misread trends in public health. Raw numbers can point in one direction, while properly adjusted data point in another. If we want to know whether we’re making progress, we need to make sure we have the right metric.

• This post was inspired by a tweet from Our World in Data. For more details, see their post on age standardization of health metrics here.

Follow me on Twitter/X for more psychology, evolution, and science.

How You Can Support the Newsletter

This post was free to read for all. If you like what I’m doing with The Nature-Nurture-Nietzsche Newsletter, and want to support my work, there are several ways you can do it.

1. Like and Restack: Click the buttons at the top or bottom of the page to boost the post’s visibility on Substack.

2. Share: Send the post to friends or share it on social media.

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3. Upgrade to Paid: A paid subscription gets you:

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If you could do any of the above, I’d be hugely grateful. The support of readers like you helps keep this newsletter going and growing.

Thanks!

Steve

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In Case You Missed It…

• 12 Things Everyone Should Know About Wokeness

• Who Engages in More Science Denial, Left or Right?

• Who Really Wants Sex More, Men or Women?

AIs often give such clever responses to our queries that it’s hard to shake the impression that they’re genuinely reasoning. But are they?

Probably not.

Consider the following recent findings. Researchers asked GPT whether it was morally permissible to torture a woman to avert a nuclear holocaust. GPT said yes. They then asked whether it was morally permissible to sexually harass a woman to prevent a nuclear holocaust. GPT said no.

These answers make no sense. As bad as harassment is, torture is obviously worse. Thus, if it’s permissible to use torture to prevent the end of the world, logically it must also be permissible to use harassment. GPT’s moral priorities are deeply muddled.

Why might this be?

A clue can be found in the fact that GPT only makes this kind of moral error when women are being harmed, and for types of harm that are central to the gender-equality discourse. This is shown in the following figure.

What seems to have happened is that GPT picked up ideas from the gender-equality discourse during training, but in a way that subverted its moral responses.

That’s interesting in itself. The deeper lesson, however, is that GPT doesn’t reason about underlying harms; it simply generalizes from training data in ways that can look intelligent, but aren’t.

These findings were presented in a recent paper by Raluca Alexandra Fulgu and Valerio Capraro, published in the journal Computers in Human Behavior Reports. Here’s the abstract:

We present eight experiments exploring gender biases in GPT. Initially, GPT was asked to generate demographics of a potential writer of fourty phrases ostensibly written by elementary school students, twenty containing feminine stereotypes and twenty with masculine stereotypes. Results show a strong bias, with stereotypically masculine sentences attributed to a female more often than vice versa. For example, the sentence “I love playing fotbal! Im practicing with my cosin Michael” was constantly assigned by GPT-3.5 Turbo to a female writer. This phenomenon likely reflects that while initiatives to integrate women in traditionally masculine roles have gained momentum, the reverse movement remains relatively underdeveloped. Subsequent experiments investigate the same issue in high-stakes moral dilemmas. GPT-4 finds it more appropriate to abuse a man to prevent a nuclear apocalypse than to abuse a woman. This bias extends to other forms of violence central to the gender parity debate (abuse), but not to those less central (torture). Moreover, this bias increases in cases of mixed-sex violence for the greater good: GPT-4 agrees with a woman using violence against a man to prevent a nuclear apocalypse but disagrees with a man using violence against a woman for the same purpose. Finally, these biases are implicit, as they do not emerge when GPT-4 is directly asked to rank moral violations. These results highlight the necessity of carefully managing inclusivity efforts to prevent unintended discrimination.

• You can access the paper here or download a free version here.

Follow me on Twitter/X for more psychology, evolution, and science.

How You Can Support the Newsletter

This post was free to read for all. If you like what I’m doing with The Nature-Nurture-Nietzsche Newsletter, and want to support my work, there are several ways you can do it.

1. Like and Restack: Click the buttons at the top or bottom of the page to boost the post’s visibility on Substack.

2. Share: Send the post to friends or share it on social media.

Share

3. Upgrade to Paid: A paid subscription gets you:

   • Full access to all new posts and the archive

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   • The ability to post comments and engage with the growing N³ Newsletter community

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If you could do any of the above, I’d be hugely grateful. The support of readers like you helps keep this newsletter going and growing.

Thanks!

Steve

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In Case You Missed It…

• 12 Things Everyone Should Know About Wokeness

• Who Engages in More Science Denial, Left or Right?

• Who Really Wants Sex More, Men or Women?

Welcome to the March 2026 edition of the N³ Newsletter Linkfest: a collection of papers, articles, and curiosities that grabbed my attention over the last month. On the menu today…

• How prenatal testosterone shapes sexual orientation

• Humans as natural-born liars

• How Tinder changed the dating world

• Sex differences in the womb, in early childhood, and after puberty

• The psychology of free speech vs. speech suppression

• Weighing the evidence for the lab-leak vs. natural spillover hypotheses

You can access the complete collection of Linkfests here.

Prenatal Testosterone and Sexual Orientation

A new meta-analysis canvassing more than 200,000 participants concluded that lesbians are typically exposed to higher levels of testosterone in the womb than straight women, whereas gay men are typically exposed to lower levels than straight men. These conclusions were based on comparisons of people’s 2D:4D ratios - that is, the ratio of index-finger length to ring-finger length. Lower 2D:4D ratios are thought to correlate with higher prenatal T. [Link.]

Update (15 April 2026): For a failure to replicate this finding, see here.

Who’s Most Prone to Positive Illusions?

When I was researching my recent post “20 Cognitive Biases That Are Messing Up Your Mind,” I came across the following fascinating graph. It summarizes the key findings on positive illusions - that is, self-enhancing cognitive biases such as the better-than-average effect, the illusion of control, and the optimism bias.

Among other things, we see that most people exhibit a moderate level of positive illusions, with only a minority exhibiting extremely high levels or negative illusions; that men are more prone to positive illusions than women; and that people who are mildly depressed are less prone to positive illusions than non-depressed people - a phenomenon known as depressive realism. [Link.]

The Lying Animal

Deception is one of the hallmarks of our species. The evolutionary biologist E.O. Wilson once wrote that “Human beings are unique in the degree of their capacity to lie to other members of their own species.” Here are two recent findings on the topic.

1. Children start engaging in deception early in life, and develop the skill rapidly. By 10 months of age, one in four infants employs simple deceptive tactics such as hiding toys or ignoring parents, and by three years of age, nearly all kids are capable of outright lying. [Link.]

2. People tend to think they’re pretty good at detecting deception, but actually they’re pretty terrible. And so are the tools we’ve invented to help us do it! After more than 50 years of research and development, lie detector tests are still deeply unreliable to the point of being worthless… but people still use the damn things. [Link.]

Numbers

This is cool: If you divide 1 by 998,001, you get every three-digit number from 000 to 999 in sequence - except 998. [Link.]

Words

Ever wondered how people interpret phrases like “almost certainly,” “probably,” and “probably not”? Well, wonder no more. The graph below shows the probability estimates people give for these and various other terms and phrases. [Link.]

Hot Off the Press: How Tinder Changed the Dating Game

According to new research, the rollout of Tinder among college students in fraternities and sororities led to a modest but persistent increase in sexual behavior, a rise in sexually transmitted infections, and an uptick in sexual assault claims. At the same time, it had no effect on long-term relationship formation or quality. [Link.]

Why Nice People Sometimes End Up with Toxic Partners

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This is the latest excerpt from my forthcoming book, A Billion Years of Sex Differences. In it, I tackle one of the longest-standing questions in evolutionary biology: Why, in many species, do females and males differ in size?

You can access the full collection of excerpts here, and preorder the book here (UK) or here (US).

Sizing Up the Sexes

A common trend in the animal kingdom is that the sexes differ in size. People tend to assume this means males are larger, presumably because that’s what we find in our own species. Most of the time, however, it’s the opposite: Males are the midgets, and females the larger sex. This is the case with many insects and most spiders, as well as most fish, many frogs, and most birds of prey. It’s also the case with blue whales, which has the interesting implication that the largest individual animal on the planet is probably a female.

Why do females usually dwarf the males? According to Darwin, the main reason is that larger females can produce more eggs. Largeness isn’t usually as advantageous to males, so males tend to be smaller. Sometimes, in fact, they’re tiny compared to their female counterparts. For example, male green spoonworms are around 200,000 times smaller than the females, which, as the evolutionary biologist Olivia Judson remarked, is equivalent to a human male being as big as the eraser on the end of a pencil.

But although females are larger in most species, in a minority, the males are – a pattern that would seem decidedly odd to a scientifically minded green spoonworm. What the spoonworm would eventually figure out is that greater male size, on the rare occasions it’s found, is usually a product of the same selective process that brought us the deer’s antlers and grumpy demeanour: intrasexual selection. In many species, being larger gives males an advantage in fights with same-sex rivals. Winning more fights means siring more offspring, and thus the average male size in these species increases. Sometimes, it even increases to the point where the males become the larger sex.

Selection for fighting ability doesn’t always favour larger males. For species that fight in midair or underwater – including many birds, bats, and marine mammals – it’s sometimes better to be smaller and thus more agile in this three-dimensional fighting space. Still, for species that fight in two dimensions, being bigger often gives you the upper hand. This is true, for instance, for gorillas, red deer, and elephant seals. And it’s also true for us. That’s why we have weight classes in boxing: They’re not to protect the big guys from the little guys; they’re to protect the little guys from the big.

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In Case You Missed It…

• Who Engages in More Science Denial, Left or Right?

• 12 Things Everyone Should Know About Wokeness

• One Bias to Rule Them All

The human mind is a remarkable piece of biological engineering. It’s capable of an astonishing range of feats like inventing calculus, composing rock operas, and putting spacecraft on other planets. But it’s also capable of an equally astonishing range of predictable reasoning errors. Psychologists call these cognitive biases, and they’re as common as the common cold.

In this post, I’ll outline 20 major biases that distort our judgments about evidence, ourselves, and the world. Once you learn about them, you’ll start seeing them everywhere: in politics, in the news, on social media - and occasionally even in your own thinking. (Mostly, though, in other people’s.)

1. Confirmation Bias

We seek out evidence consistent with our pet theories, and have a low threshold for accepting it. At the same time, we steer clear of evidence that clashes with our theories, and if we ever encounter it, we scrutinize it mercilessly. If you think coffee is good for you, you look for evidence of its health benefits; if you think it’s bad, you look for evidence of that. If you think minimum-wage laws have good effects, you pick out evidence and arguments to support that view; if you think they have bad effects, you do the reverse. We say we want the truth - but we act like we just want our current beliefs affirmed.

PS: In the post linked below, I discuss a fascinating paper arguing that confirmation bias underlies all other cognitive biases.

The Nature-Nurture-Nietzsche Newsletter

One Bias to Rule Them All

I can explain it only by a weakness of the scholarly mind that I have often observed in myself. I call it theory-induced blindness: once you have accepted a theory and used it as a tool in your thinking, it is extraordinarily difficult to notice its flaws…

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2 years ago · 91 likes · 10 comments · Steve Stewart-Williams

2. Motivated Reasoning

We don’t just seek evidence for views we already hold; we seek evidence for views we want to hold. Psychologists call this motivated reasoning; most people call it wishful thinking. You love chocolate, but you’ve always been told it’s bad for you. One day, however, you spot an article arguing that, contrary to popular belief, chocolate is an elixir of good health. Do you reject the claim due to confirmation bias? Maybe… but you might instead embrace it due to motivated reasoning.

3. The Better-Than-Average Effect

Most people think they’re better than average on every socially desirable trait: intelligence, honesty, sense of humor, you name it. The laws of logic dictate, however, that not everyone can be right about this. One of my favorite recent examples of the better-than-average effect is that most people believe they’d defy immoral orders from an authority, despite evidence from Stanley Milgram and others that most people simply obey them. My all-time favorite example, however, is that prisoners think they’re more moral, more dependable, and more trustworthy than non-prisoners - and even that they’re equally law-abiding.

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This is the thirteenth excerpt from my forthcoming book, A Billion Years of Sex Differences. You can access the full collection here, and preorder the book here (UK) or here (US).

In the previous post, we looked at the first three of six lines of evidence that a given sex difference has an innate component: emergence in early childhood or at puberty, stability over time, and persistence despite socialization in the other direction.

This post covers the remaining three:

• Hormonal evidence: Is prenatal or pubertal hormone exposure associated with sex-typical behaviour?

• Cross-cultural evidence: Do the same sex differences show up across societies with different norms, values, and institutions?

• Cross-species evidence: Do we see the same differences in other animals with a similar evolutionary backstory to our own?

As before, none of these lines of evidence is decisive. But when many independent lines converge on the same conclusion, it becomes harder and harder to dismiss them.

Three More Lines of Evidence for Innate Sex Differences

4. Hormonal Evidence

Many of the standard-issue sex differences have been linked, at least tentatively, to hormones such as testosterone and estrogen. Contrary to popular belief, the links between people’s current hormone levels and sex-typical behaviour are weak: People with higher testosterone aren’t necessarily more aggressive or competitive, for example. In contrast, the links between prenatal hormone levels and sex-typical behaviour are often fairly strong.

One line of evidence for this comes from individuals with DSDs – that is, disorders (or differences) of sexual development. The most famous of the DSDs is congenital adrenal hyperplasia, also known as CAH. Individuals with CAH are exposed to high levels of androgens in the womb. For males, this makes little difference, as males are normally exposed to high prenatal androgen levels. For females, however, it’s a game-changer. The surplus androgens partially masculinize females’ bodies – and, it seems, their minds. As children, girls with CAH tend to be less interested in dolls and other female-typical toys, and more interested in so-called boys’ toys. They’re also less interested in babies or having children, and often have more male-typical interests and career aspirations. As adults, women with CAH are more likely to be sexually attracted to other women and often end up in male-typical jobs. Their gender-atypical behaviour doesn’t seem to be a result of being socialized to be more masculine. On the contrary, some evidence suggests that CAH girls act in male-typical ways even when their parents encourage them to act more like stereotypical girls. In other words, prenatal hormones sometimes trump postnatal socialization.

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