this is a deceptively difficult question. most people's first answer, i think, would be "well, it's biology". but the situation there is actually incredibly vague. this is fairly long, but it's just going into the details to clear up every point - if you skip to the last section, there's a quick read version too.
The biological and medical concept of sex
so, if we look for "what makes men and women different" in biology, we need to be specific about what exactly are the biological differences between men and women. and here, most people would go for a straightforward, simple answer - "well, you've got a dick, or you've got a vagina. it's not hard."
unfortunately, accidents can happen - any amount of things can happen in the course of day to day life that renders your genitals either useless, or, in more dramatic cases, ruined to the point of not even being there (not even getting into the realm of birth "defects"). but nobody would assert that a man who'd lost his genitals in an accident stopped being a man. so, as a conesquence, we can't really say "having a dick or a vagina makes you male or female", because there are men without penises, and women without vaginas, for various reasons. i've kind of glossed over the topic of things from birth, because it's more complicated, and we'll get into it later - but suffice it to say, we need to look elsewhere for the biological thing that determines sex.
so, some people would say "well, it's your brain - you have a male brain, or a female brain." the general argument goes, "men and women have differences in their brain that are inherent, and hardwired, that makes them male or female." if you go from this, you neatly avoid the problems of going by genitals, and you aren't lead to unwanted conclusions like "a man who loses his penis in an accident stops being a man and starts being some weird genderless nothing". it might be surprising, given that you can cite so many studies that seem to support this, but this isn't on the most solid ground either.
Experts used to attribute gender inequality to the “delicacy of the brain fibers” in women ; then to the smaller dimensions of the female brain (the “missing five ounces,” the Victorians called it); then to the ratio of skull length to skull breadth. In 1915 the neurologist Dr. Charles L. Dana wrote in this newspaper that because a woman’s upper spinal cord is smaller than a man’s it affects women’s “efficiency” in the evaluation of “political initiative or of judicial authority in a community’s organization” — and thus compromises their ability to vote.
These days gender inequality is commonly explained by neurological differences, most popularly the notion that the surge oftestosterone that occurs in the eighth week of fetal development affects the relative size of the right and left hemispheres of the brain, and of the corpus callosum, the bundle of neurons that connects the two. In the 1980s
this quote here is taken from a summary of cordelia fine (a cognitive neuroscience phd)'s book Delusions of Gender: How Our Minds, Society, and Neurosexism Create Difference. what it's about, essentially, is that all those studies you could cite, are pretty crap for a wide variety of differences. john ioannidis's famous 2005 paper on why most medical research papers were false is relevant too.
Norman Geschwind proposed that the surge results in a smaller left hemisphere for males, leaving them with greater potential for right-hemisphere development, which, as he put it, results in “superior right-hemisphere talents, such as artistic, musical, or mathematical talent.” In female brains the hemispheres are more collaborative, explaining women’s superior verbalizing skills.
There are two problems here, Dr. Fine says. First is that several studies have found no difference in hemispheric size in neonates. The supposedly larger female corpus callosum is also in dispute. But even if size difference does exist (as it does in rats), she says, “getting from brain to behavior has proved a challenge.” Given that there may be sex differences in the brain, “what do they actually mean for differences in the mind?”
or to summarize her point on the supposedly best done studies on this matter,
Nonexistent sex differences in language lateralization, mediated by nonexistent sex differences in corpus callosum structure, are widely believed to explain nonexistent sex differences in language skills.
and in the same vein, we have Brainstorm by Rebecca Jordan-Young on largely the same topic, doing largely the same job - showing why those studies pretty much fail.
meanwhile, the much vaunted absolute psychological differences between men and women don't really materialize as such. as shown by this, http://www.rochester.edu/news/show.php?id=5382 and many others.
so, while this is a matter that really does deserve books written on it (luckily, they have been, and you should read them if you're skeptical that there are no inherent "hardwired" brain differences), we still have another problem - it doesn't look like we can use "hardwired" brain differences to explain why men and women are different.
now, the next answer, and to some of you, your first answer, is "well, it's in the dna." or put more specifically "your karyotype - whether you have an xx chromosome, or an xy chromosome. xx is female, xy is male, and it's as simple and absolute as that."
this is maybe the best candidate for identifying something as making someone male or female, it might seem - it's absolute, it's set from birth, it doesn't change in accidents or messups, we can determine it clearly with no ambiguity, we believe that all other sex differences simply follow from karyotype, and we know that they exist, unlike brain sex differences. it's an open and shut case in favour, right? in fact, even most doctors would tell you something along these lines. surely there's no better support than that, right?
you'd be surprised.
The history of the medical concept of sex
there's a very long history of biologists in past centuries trying to come up with biological excuses to claim that women were, in some sense, inferior to men. now, in this section, i'm going to borrow fairly extensively from people who've written about this topic much better than i have. let's start with A Gendered Critique of Sex Determination Hypotheses.
- "Throughout much of European thought, women were equated with "lower" races and white children. They had not developed fully. Evolutionists Thomas Huxley, Ernst Haeckel, E. D. Cope, and Herbert Spencer still held the Aristotelian notion that women were like men, only that their development or evolution had been truncated (see Sayers, 1982)."
however, the simplistic theories had a big wall to get over, and so do any theories really. the existence of intersex people.
intersex people, are, in essence, what it sounds like - they are intersex. typically, this is what happens when doctors get a child - they look at the genitals, they say "it's a boy" or "it's a girl". but when a doctor receives a child with indeterminate or difficult genitals, they call them intersex, because they can't really make an immediate assessment. the isna (intersex society of north america) puts it as "Intersex is a general term used for a variety of conditions in which a person is born with a reproductive or sexual anatomy that doesn’t seem to fit the typical definitions of female or male."
- "In our work, we find that doctors’ opinions about what should count as “intersex” vary substantially."
it throws a spanner in the wrench for the 1900s, that's for sure.
In 1843, Levi Suydam, a 23 year old resident of Salisbury, Connecticut, asked the town magistrates to validate his right to vote as a Whig in a particularly divisive local election. The opposition party raised objections, saying that Levi was really a woman and therefore unable to vote. A doctor examined him and declared that he had a penis and was therefore a man. Duydam voted and the Whig candidate won by a single vote. Within a few days after the election, Suydam had his monthly menstrual bleeding (Fausto-Sterling, 1993). Hugh Young (1937) relates that one of his patients, a hermaphrodite named Emma had a penis-sized clitoris and a vagina. Raised as a girl, she could have "normal" heterosexual relationships with both men and women. And she did. She functioned sexually as both male and female all her adult life.
However, as medicine became more sophisticated, it decided that society did not tolerate hermaphrodites and parents wanted their babies to be either male or female. In the early 1900s, "true" sex was said to be the sex of the gonads. Thus, people having androgen insensitivity syndrome were classified as "really" males, even though their entire physical appearance and usually their sexual orientation was female (Dreger, 1998). It was thought that society could not deal with people who were not either one or the other sex. (And, as Dreger demonstrates, this was stated explicitly by several physicians). Our birth registries still demand that a newborn be quickly placed into one or the other category, and in the early 1900s, knowledge of the "true" sex of a person was thought to be critical to prevent inadvertent homosexual relations. Our current classification scheme of male and female pseudohermaphrodites reflects this gonadal (and later, chromosomal) assignment of sex.
- "Starting in the 1960s, babies born with ambiguous genitalia were "assigned" a sex that seemed appropriate based on the genitalia that they had. Those with large phalluses had their labia closed and became males, while those with smaller but still larger than normal phalluses had them surgically shortened and became females. In the 1990s, some of the individuals who were surgically assigned their sex founded the Intersex Society of America (ISNA) and lobbied to speak to physicians to have them change their usual practice of surgically amending nature. Their arguments convinced many physicians that having a baby of ambiguous sex was not a medical emergency, that interventions should be reversible, and that time should be taken to discuss these issues with parents and patients with such conditions. "
for a very long time, doctor's had simple measurements they used to decide whether they would determine a baby's genitals, not because there was any medical need, but simply because, they had to be conformed into an easy and recognizable male or female category, and not because this absolute binary existed in nature. in a very real sense, at this point, biological sex differences were being created by doctors.
as alice dreger (author of Hermaphrodites and the Biomedical Invention of Sex) has written for the ISNA,
- "I came to this topic as an historian and philosopher of science. My initial interest was actually in learning how British and French medical and scientific men of the late nineteenth century dealt with human hermaphroditism. The late nineteenth century was a time when the alleged naturalness of European social sex borders was under serious challenge... I wanted to know what biomedical professionals did, at such a politically charged time, with those who inadvertently challenged anatomical sex borders.
The answer is that biomedical men tried their best to shore up the borders between masculinity and femininity. Specifically, the experts honed in on the ovarian and testicular tissues and decided that these were the key to any body’s sexual identity. The “true sex” of most individuals thus by definition settled nicely into one of the two great and preferred camps, no matter how confusing the rest of their sexual anatomies. People with testicular tissue but with some otherwise “ambiguous” anatomy were now labeled “male pseudo-hermaphrodites”that is, “true” males. People with ovarian tissue but with some otherwise ambiguous anatomy were labeled “female pseudo-hermaphrodites”“true” females."
- "By equating sex identity simply with gonadal tissue, almost every body could be shown really to be a “true male” or a “true female” in spite of mounting numbers of doubtful cases. Additionally, given that biopsies of gonads were not done until the 1910s and that Victorian medical men insisted upon histological proof of ovarian and testicular tissue for claims of “true hermaphroditism,” the only “true hermaphrodites” tended to be dead and autopsied hermaphrodites.
Nevertheless, new technologies-specifically laparotomies and biopsies-in the 1910s made this approach untenable. It now became possible (and, by the standing rules, necessary) to label some living people as “true” hermaphrodites via biopsies, and disturbed physicians noted that no one knew what to do with such people. There was no place, socially or legally, for true hermaphrodites.Moreover, physicians found case after case of extremely feminine-looking and feminine-acting women who were shown upon careful analysis to have testes and no ovaries."
In the 1910s, physicians working with intersexuality realized that assigning these women to the male sex (because of their testes) or admitting living true hermaphrodites” (because of their ovotestes) would only wreak social havoc. Consequently, in practice the medical profession moved away from a strict notion of gonadal “true sex” toward a pragmatic concept of “gender” and physicians began to focus their attentions on gender “reconstruction.”Elaborate surgical and hormonal treatments have now been developed to make the sexual anatomy more believable, that is, more “typical” of the gender assigned by the physician.
Many parents, especially those unfamiliar with sex development, are bothered by their children’s intersexed genitals and receptive to offers of “normalizing” medical treatments. Many also actively seek guidance about gender assignment and parenting practices. In the United States today, therefore, typically upon the identification of an “ambiguous” or intersexed baby teams of specialists (geneticists, pediatric endocrinologists, pediatric urologists, and so on) are immediately assembled, and these teams of doctors decide to which sex/gender a given child will be assigned. A plethora of technologies are then used to create and maintain that sex in as believable a form as possible, including, typically, surgery on the genitals, and sometimes later also on other “anomalous” parts like breasts in an assigned male; hormone monitoring and treatments to get a “cocktail” that will help and not contradict the decided sex (and that will avoid metabolic dangers); and fostering the conviction among the child’s family and community that the child is indeed the sex decided”psychosocial” rearing of the child according to the norms of the chosen sex. Doctors typically take charge of the first two kinds of activities and hope that the child’s family and community will successfully manage the all-critical third.
and here we come to something much more difficult that we won't get into, but suffice it to say, the search for an absolute notion of where you can cut someone off as male and female didn't end at the 1900s, and all those answers had ways of contradicting each other.
a quick note on karyotype before we go on
even karyotype can't determine if you're male or female. there are women with xy chromosomes - the y chromosome being the one that contains the sry gene, and therefore, is supposed to create testicular development - and there are men with two x chromosomes. they go through life completely normally and treated unquestionably as men and women, but their karyotypes aren't what people would reference as male or female karyotypes. that's not even getting into the difficult cases with o chromosomes, with mosaics, and etc.
these aren't absolute stones that your gender is carved onto - these are little blobs of chemicals. their only relevance to your biological sex is what they did while you were in the womb, and after that, they're nothing. you could remove a y chromosome from a man - assuming you had incredibly sophisticated future technology - and he'd still have a penis, he'd still have testicles, he'd still have a scrotum, he still wouldn't have a uterus, he'd still think he was a man, act like a man, and talk to others like he was a man, and be treated as a man.
dna, just being there, means nothing. you could splice lizard or fly or whatever dna you wanted into your body, if it doesn't get expressed (which is what would actually happen if you did this), it would have no real consequences. you wouldn't grow fly limbs or wings.
If you're skipping to the end because it's long, this is the part you were meant to skip to.
so, let's put a lot of this together.
what are the actual ways we'd say men and women are biologically different? the conventional answers would be: their genitals (penis? vagina?), their gonads (ovaries? testicles?), their karyotype (xy? xx?), the dominant hormones (androgens (like testosterone)? estrogen?) and the rest we'd call secondary sex characteristics. this means, for example, female skin tone, presence of higher muscle mass, arrangement of fatty tissues, growth of bone structure, bone density, and even whether or not you have boobs. seriously, anyone could go grab some anti androgens, take estrogen for a few months, and they would grow boobs. they're a secondary sex characteristic.
all secondary sex characteristics are produced by the hormone levels of either androgens or estrogen. as a matter of fact, even genital formation - whether or not a penis forms - is determined by the presence of androgens or estrogen in the womb. the differences that form during puberty, again, a product of hormones (androgens, estrogen). hormone replacement therapy for transgender people is sometimes likened to a second puberty in light of this (and the effects of hrt can be quite dramatic - everything changes. have a look at some before and after photos. seriously). and these hormones are produced by the gonads. and supposedly, the gonads are formed by the karyotype.
but in biology, things are rarely this simple. karyotypes can mess up. the formation of genitals or gonads sometimes becomes weird for no reason. someone can have an xy karyotype but be completely insensitive to androgens, and thus simply develop into a woman with testicles, who doesn't menstruate, and has high levels of testosterone. genitals can vary. hormone levels vary all the time at the slightest touch. it's all interconnected and can simply just randomly go one way or the other.
thus, we're left with a problem: if we identify the thing that makes men and women different as karyotype, we'd call a lot of women men for no reason, and a lot of men women. if we identified it with gonads, we'd make the same mistake. we can't say genitals, because many women and men don't have a penis or vagina or other expected genitals but still are uncontroversially treated by everyone they know as men or women. if we identify it as hormone levels, these can change for all kinds of health reasons, and so we're left with people who are men or women, and then for a few weeks, stop being men and women, but go back to being men and women again. we've got no way out.
the only real option, is essentially this.
biological sex characteristics are real things, but the categories we arrange them into (male and female) are not absolute realities in nature, but rather, ones that humans constructed. there is no absolute barrier in nature, just as there's no absolute barrier between red and orange, but we still have words for red and orange.
in other words, biological sex is not simply two binary options, but a cluster or spectrum of possibilities, that is, it seems, not quite correlated with gender. i'm just about out of space, so i'll go into the issue of gender another time. and also, actually answer the question in the title.