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A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women
According to an old joke, everyone knows that men and women are different... except social scientists. In A Billion Years of Sex Differences, evolutionary psychologist Steve Stewart-Williams tackles some of science's most controversial How do men and women differ? Where do the differences come from? And how do they shape modern life?
The result is the most up-to-date, balanced and engaging account of human sex differences yet written, covering everything from dating and mating to aggression and parenting, from children's toy preferences and workplace gender gaps to mental health and the politics of equality. Drawing on a century of research - and a billion years of evolution - Stewart-Williams explains why many sex differences appear despite socialization, not because of it; why in our mating and parenting patterns, humans are more like the average bird than the average mammal; and why sex differences are sometimes a sign of societal health rather than injustice.
With more than two decades of research in evolutionary psychology and an international reputation in the field, Steve Stewart-Williams is uniquely positioned to challenge prevailing views in this fraught debate. Avoiding the extremes of either overstating or denying the differences, he argues that, rather than trying to make men and women identical, we should strive for a world where bias and barriers are eliminated, and where people are free to be themselves whether they conform to gender norms or defy them.
The result is the most up-to-date, balanced and engaging account of human sex differences yet written, covering everything from dating and mating to aggression and parenting, from children's toy preferences and workplace gender gaps to mental health and the politics of equality. Drawing on a century of research - and a billion years of evolution - Stewart-Williams explains why many sex differences appear despite socialization, not because of it; why in our mating and parenting patterns, humans are more like the average bird than the average mammal; and why sex differences are sometimes a sign of societal health rather than injustice.
With more than two decades of research in evolutionary psychology and an international reputation in the field, Steve Stewart-Williams is uniquely positioned to challenge prevailing views in this fraught debate. Avoiding the extremes of either overstating or denying the differences, he argues that, rather than trying to make men and women identical, we should strive for a world where bias and barriers are eliminated, and where people are free to be themselves whether they conform to gender norms or defy them.
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First published June 4, 2026
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Displaying 1 - 4 of 4 reviews
June 4, 2026
Steve Stewart-Williams’ A Billion Years of Sex Differences is one of the most important books on sex differences published in recent years. Drawing on evolutionary psychology, biology, anthropology, genetics, and cross-cultural research, Stewart-Williams takes readers on a fascinating journey through the deep evolutionary roots of human nature.
What makes this book especially valuable is its balanced and evidence-based approach. Rather than viewing men and women through an ideological lens, Stewart-Williams examines the scientific evidence with curiosity, humility, and rigour. He rejects simplistic claims that men and women are either completely the same or entirely different, instead exploring where sex differences are large, where they are small, and why they exist.
Importantly, this is neither a male-negative nor female-negative book. It honours both masculine and feminine traits, recognising that each sex brings distinctive strengths, vulnerabilities, preferences, and contributions. The result is a deeply human account of who we are and how we came to be.
Readers may disagree with some conclusions, but they will struggle to find a more comprehensive synthesis of the evidence. In an era where discussions of sex differences are often dominated by politics and ideology, A Billion Years of Sex Differences stands out as a thoughtful, respectful, and scientifically grounded contribution.
Whether you are interested in psychology, relationships, parenting, education, or public policy, this book deserves a wide readership. It reminds us that understanding men and women begins not with slogans, but with evidence.
What makes this book especially valuable is its balanced and evidence-based approach. Rather than viewing men and women through an ideological lens, Stewart-Williams examines the scientific evidence with curiosity, humility, and rigour. He rejects simplistic claims that men and women are either completely the same or entirely different, instead exploring where sex differences are large, where they are small, and why they exist.
Importantly, this is neither a male-negative nor female-negative book. It honours both masculine and feminine traits, recognising that each sex brings distinctive strengths, vulnerabilities, preferences, and contributions. The result is a deeply human account of who we are and how we came to be.
Readers may disagree with some conclusions, but they will struggle to find a more comprehensive synthesis of the evidence. In an era where discussions of sex differences are often dominated by politics and ideology, A Billion Years of Sex Differences stands out as a thoughtful, respectful, and scientifically grounded contribution.
Whether you are interested in psychology, relationships, parenting, education, or public policy, this book deserves a wide readership. It reminds us that understanding men and women begins not with slogans, but with evidence.
June 14, 2026
A clear and lucid account of the science and debate involving the differences between men and women. It is a well-balanced account of the arguments and evidence. The upshot is that men and women do differ on average, but that the differences are usually modest. In comparison with other animals, we seem to occupy a somewhat awkward position where sexual differences are significant but do not entirely dominate our behavior. The author concludes that while the behavioral and social differences are significantly modulated by culture, there is an innate component that is difficult to explain away.
June 9, 2026
قد تتحول الصور النمطية الجنسية إلى نبوءات تحقق ذاتها. فعلى سبيل المثال، إذا افترض الأهل والمعلمون أن الرياضيات والعلوم مخصصة للأولاد، بينما القراءة والكتابة مخصصة للفتيات، فقد يغفلون عن ملاحظة أو تنمية مواهب الأطفال في المجالات التي تتعارض مع توقعاتهم. وفي الوقت نفسه، إذا اعتقد الصبيان أنهم لن يكونوا بارعين في رعاية الأطفال، أو الفتيات أنهن لن يكنّ بارعات في التعامل مع الأرقام، فقد يقللون من الوقت والجهد المبذولين في هذه الأنشطة، مما يضمن في النهاية عدم تفوقهم فيها. وحتى لو اعتقد الأطفال أنهم قادرون على التفوق في مجال ما، فإنهم لا يرغبون في الظهور بمظهر غريب، خاصة في سنوات المراهقة، لذا غالبًا ما يحصرون أنشطتهم اليومية وطموحاتهم الحياتية في ما يُعتبر طبيعيًا لجنسهم.
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Steve Stewart-Williams
A Billion Years of Sex Differences
Translated By #Maher_Razouk
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Steve Stewart-Williams
A Billion Years of Sex Differences
Translated By #Maher_Razouk
July 1, 2026
This book is great and shows how evolutionary psychology and biology have improved a lot in the last years. We have left behind the hard, dogmatic and deterministic discourses of Dennett or Dawkins, and also the little nuanced reductionisms of Pinker or Buss. Stewart-Williams manages to nuance, explain the difference between statistical generalities and individual or group realities and, simply, go to the biological root of the question. In this sense, this is an outstanding piece of work and very recommendable. A MUST for philosophers, sociologists, psychologists and so on.
Now then, there are several criticisms to make, even if they are moderate...
The first one is that the author has a perception of "culture" that is a bit poor (while he has a very good perception of biology and science), which is normal. What the author obviates—or is not capable to emphasize—is that all biology is crossed by words and stained by culture. Every fetus emerges already in a human amniotic fluid, and vice versa. The maternal instinct itself, which is undeniable, takes many forms, nuances, hates, obsessions, negations, depressions... precisely because of human culture. And I am not denying that in animals the mothers can also do infanticide because of stress; I say that the way of living it, giving it a meaning, etcetera, is culture.
However, Stewart-Williams, in his bias, pulls always more towards the biological side, as if the match was played there. And of course, it is not like that. Although he says that "Nature's contribution is more crucial", in the last chapter he cannot say much about how to "solve" our gender problems. To do that, one has to speak about politics, anthropology, education, symbolisms, marketing, art... about social sciences, which he, like so many evolutionary authors, see as enemies.
The author lacks a more processual biological vision in the line of John Dupré or David Noble, by levels of meaning and development, where sex is not only seen in gametes (which is true at a biological level), but also at the cultural, social, human and symbolic phenomenological level where WE move. Organs, phenotype... everything is key in the human dimension where we live. As many authors have claimed: "sex in humans is, overally speaking, about brains and MIND". But Stewart-Williams tries to circumnavegate this two huge icebergs...
To put an example of what we mean about what should be nuanced or what the author "lacks": Does a space satellite owe more to biology or to culture? And a sculpture finely made on a grain of rice? Of course, without creative beings we wouldn't have the satellite, and without rice we wouldn't have the sculpture. BUT, the actual truth is that a huge part of society—and of history—has not had satellites. And, if we want to understand the sculpture artistically and anthropologically, that it is made of rice or clay can be the less important thing.
The defect that I think the author should check the most is the religious belief he has in "The second law of behavior genetics". He should review the scientific criticisms to twin studies (as, for example, Sapolsky collects in Behave or Determined). There are many scientific authors who have turned around this simplistic vision of the effect of families on brothers. To cite some: Jay Joseph, Leon Kamin, Marcus Feldman, Evan Charney and... Eric Turkheimer, the creator of this "law" who at the end criticized it! I point out by the way that there are also certain psychological limits. The author has it taken with social scientists and psychologists; I understand this is because they have often undervalued the real differences of biology. But it is also true that the author simplifies and underestimates the psychological nuances and depth.
Of course, I have pointed out many things that seem to me he should nuance or that are naive. Yes, there is a lot of naivety, because I think that often those who only opt for a biologizing perspective believe that everything is very simple, and the problems get erased. What is more, Stewart-Williams also should acknowledge that when he addresses the topic of homosexuality, he still has a long way to go in terms of reviewing and reflecting (what he points out is not bad, but again he wants to consider it a closed case with certain reductionist possibilities). And, of course, the trans topic, which is very key, is not even mentioned.
For this reason, I would also recommend, to complete the perspective of this author, other approaches that don't necessarily deny the fundamental thesis of the book, but make see how there are other dimensions to take into account if someone really wants to treat the theme of sex differences in all its human complexity. I refer to books like Sexual Ambiguities by Geneviève Morel, El deseo trans by Vilma Coccoz, Deshumanizando al varón by Daniel Jiménez, Righteous Men by Ivan Jablonka, A la conquista del cuerpo equivocado by Miquel Missé, or ¿Existe la relación sexual? by Massimo Recalcati... all of them very different and diverse, and of course none of them perfect (and I have left many others in the inkpot). Also is highly recommendable to read Cordelia Fine, because, as Stewart Williams put it "She writes very well", what is true (although some of hers arguments are tricky).
Anyway, I have to stress again that this essay is a MUST. No debate or discussion will be possible (in philosophy, sociology or whatever) without taking into account the basis scientific facts that Steve Stewart-Williams show us here.
Now then, there are several criticisms to make, even if they are moderate...
The first one is that the author has a perception of "culture" that is a bit poor (while he has a very good perception of biology and science), which is normal. What the author obviates—or is not capable to emphasize—is that all biology is crossed by words and stained by culture. Every fetus emerges already in a human amniotic fluid, and vice versa. The maternal instinct itself, which is undeniable, takes many forms, nuances, hates, obsessions, negations, depressions... precisely because of human culture. And I am not denying that in animals the mothers can also do infanticide because of stress; I say that the way of living it, giving it a meaning, etcetera, is culture.
However, Stewart-Williams, in his bias, pulls always more towards the biological side, as if the match was played there. And of course, it is not like that. Although he says that "Nature's contribution is more crucial", in the last chapter he cannot say much about how to "solve" our gender problems. To do that, one has to speak about politics, anthropology, education, symbolisms, marketing, art... about social sciences, which he, like so many evolutionary authors, see as enemies.
The author lacks a more processual biological vision in the line of John Dupré or David Noble, by levels of meaning and development, where sex is not only seen in gametes (which is true at a biological level), but also at the cultural, social, human and symbolic phenomenological level where WE move. Organs, phenotype... everything is key in the human dimension where we live. As many authors have claimed: "sex in humans is, overally speaking, about brains and MIND". But Stewart-Williams tries to circumnavegate this two huge icebergs...
To put an example of what we mean about what should be nuanced or what the author "lacks": Does a space satellite owe more to biology or to culture? And a sculpture finely made on a grain of rice? Of course, without creative beings we wouldn't have the satellite, and without rice we wouldn't have the sculpture. BUT, the actual truth is that a huge part of society—and of history—has not had satellites. And, if we want to understand the sculpture artistically and anthropologically, that it is made of rice or clay can be the less important thing.
The defect that I think the author should check the most is the religious belief he has in "The second law of behavior genetics". He should review the scientific criticisms to twin studies (as, for example, Sapolsky collects in Behave or Determined). There are many scientific authors who have turned around this simplistic vision of the effect of families on brothers. To cite some: Jay Joseph, Leon Kamin, Marcus Feldman, Evan Charney and... Eric Turkheimer, the creator of this "law" who at the end criticized it! I point out by the way that there are also certain psychological limits. The author has it taken with social scientists and psychologists; I understand this is because they have often undervalued the real differences of biology. But it is also true that the author simplifies and underestimates the psychological nuances and depth.
Of course, I have pointed out many things that seem to me he should nuance or that are naive. Yes, there is a lot of naivety, because I think that often those who only opt for a biologizing perspective believe that everything is very simple, and the problems get erased. What is more, Stewart-Williams also should acknowledge that when he addresses the topic of homosexuality, he still has a long way to go in terms of reviewing and reflecting (what he points out is not bad, but again he wants to consider it a closed case with certain reductionist possibilities). And, of course, the trans topic, which is very key, is not even mentioned.
For this reason, I would also recommend, to complete the perspective of this author, other approaches that don't necessarily deny the fundamental thesis of the book, but make see how there are other dimensions to take into account if someone really wants to treat the theme of sex differences in all its human complexity. I refer to books like Sexual Ambiguities by Geneviève Morel, El deseo trans by Vilma Coccoz, Deshumanizando al varón by Daniel Jiménez, Righteous Men by Ivan Jablonka, A la conquista del cuerpo equivocado by Miquel Missé, or ¿Existe la relación sexual? by Massimo Recalcati... all of them very different and diverse, and of course none of them perfect (and I have left many others in the inkpot). Also is highly recommendable to read Cordelia Fine, because, as Stewart Williams put it "She writes very well", what is true (although some of hers arguments are tricky).
Anyway, I have to stress again that this essay is a MUST. No debate or discussion will be possible (in philosophy, sociology or whatever) without taking into account the basis scientific facts that Steve Stewart-Williams show us here.
Displaying 1 - 4 of 4 reviews