As a research scientist interested in gender, psychopathology, and the brain, I often lament to my research team that one can readily get a paper accepted for publication reporting a snazzy difference between men and women that people will find interesting and that will confirm preconceived notions about how different the two sexes really are. But try to get a study reporting no differences into Science, Nature, or for that matter The American Journal of Psychiatry! Such findings have no appeal at all. This issue of the Journal—with three articles on gender differences, psychopathology, and the brain—illustrates the general preference of scientists (and the reviewers who evaluate their work) for observing differences rather than similarities.

Why are studies showing gender differences so appealing?

Psychologically, there is a powerful reason. One does not need psychoanalytic training to understand the drives of the Id. Our basic biology—and the survival of our species—depends on men and women recognizing one another as different and engaging in sex-specific mating activity. Human beings pride themselves on being "higher" animals who also follow the commands of the Ego and Superego, however, and who are capable of introspection, self-awareness, altruism,intellectual creativity, and social equity. Throughout much of human history, mores concerning sexual behavior have been more flexible than would be dictated by basic biology and libidinal procreative drives. We human beings participate in and condone lifestyles that override our primitive biology, including sexual continence before marriage, celibacy, and homosexuality. Given that, our eagerness to find brain-based explanations for gender differences appears to arise more from the Id than from a healthy Ego or Superego.

Ignoring both psychodynamics and the brain for a moment, how different are men and women? The differences begin at the elemental physical level, where they are greatest, but even at this level they are relatively small and subject to many influences by social custom. For example, how easy is it, really, to tell men and women apart using body shape and facial features? There is obviously a major difference in "plumbing fixtures" (as one of my saltier male physician colleagues used to phrase it), as well as lesser differences in secondary sexual characteristics and in overall body size. There are more subtle differences in bodily shape, but these differences can be modified by clothing and exercise and often are. In fact, we have probably developed the tradition of using sex-specific clothing and hair styles in order to emphasize the physical differences in gender and make a sometimes-difficult discrimination much easier.

If one strips away the trappings of sex-differentiating apparel, puts everyone in either pants or dresses, and makes hairstyle uniform (either short or long), the distinction between men and women may not be easy, especially in young people. Elizabethan drama capitalized on this by having male actors take all the female roles (yes, Juliet, Cleopatra, and Desdemona were all played by males). In our own era we are entertained by Dustin Hoffman playing Tootsie and conversely by Barbra Streisand playing Yentl. Further, the "ideal" shape has varied over time, particularly for females. For example, soft, well-cushioned women of the Rubens type were valued in Renaissance and Neoclassical times, perhaps because food was scarce and plumpness was difficult to achieve. Contemporary lifestyles that emphasize exercise and fitness have made male and female bodies look much more similar, especially among young adults. Many young women, as well as young men, pride themselves on their leanness and "muscle definition." Human males and females resemble one another more physically than they resemble other primates, such as chimps and gorillas. Apart from primary sexual characteristics (where there is a definite dichotomy), the statistical distribution of other physical characteristics of men and women have considerable overlap. There are tall women and short men, women who have more prominent facial and body hair and men who have little, women who are angular and muscular and men who are round and pyknic. The "average woman" differs from the "average man" in a statistically predictable way, but the differences in individuals cannot be predicted.

How different are human male and female brains? Are they as different (or as similar) as human male and female bodies?

In the human brain there are no visible gross differences, as there are in the rest of the body. Does that mean that there are no differences? As pointed out in Mary Seeman's excellent overview article, the answer is probably both "yes" and "no." As the fetus differentiates into male or female, the male fetal brain is exposed to testosterone while the female fetal brain is not. Although no measurable anatomic differences are evident, the higher rates of childhood disorders such as cerebral palsy in males are considered to reflect subtle differences in brain specialization that occur as a consequence of hormonal influences. While some childhood psychiatric disorders, such as hyperactivity, could be argued to arise, in part, from parental rearing practices, "hard-wired" disorders such as cerebral palsy are almost certainly primarily brain-based. As sexual and brain maturation occur, the patterns of psychiatric and neurological disorders continue to differ slightly in men and women—although with much obvious overlap—reflecting differential hormonal effects on neuronal integrity. Other articles in this issue by Rojas et al. and by Nopoulos et al. illustrate that small measurable differences that reflect sexual dimorphism in brain anatomy may be observed, which may also explain differences in patterns and prevalence of psychopathology. Finally, in old age men and women grow more similar hormonally. They continue to display differences in patterns of disease prevalence, however, and in general females enjoy a longer life and better health, reflecting the same lesser vulnerability to physical and mental disease that was present in childhood. Apart from hormonal factors, we do not know what explains this pattern—i.e., how much is due to differences in lifestyle, and how much is driven by DNA and biology.

How different are men and women in cognitive style, in mental abilities, in personality, or in "psychology"? Are women really from Venus and men from Mars?

Here the debate becomes more heated, as the science gets more difficult to design, and the social implications become more sensitive. Most of the imputed gender differences in cognition are based on anecdote or on research that has failed to be able to correct for the confounds of societal norms and expectations. We have all heard the cliches about gender differences in cognitive style and personality over and over. Boys are more active, better at visuospatial tasks, or superior in math. Girls are more nurturant, better at language tasks, or superior in social skills. A study that examines these cliches will probably never be able to eliminate the role of social expectations. Even families or schools that claim to be "gender neutral" cannot eliminate the influence of advertising, television, or "gender biased" peers or parents. We will probably never know whether the cognitive and personality differences imputed to men and women are genuinely hard-wired (although certainly subtle and overlapping in statistical distribution) or are a consequence of social expectations and would disappear if the social expectations did not exist.

Nonetheless, here in the United States we are currently in the midst of a great social experiment. Contradicting much of human history, we are attempting to treat men and women as if they were "created equal." We are attempting to provide them with equal opportunities—in education, in the workplace, and in leadership opportunities. Medical school classes now accept nearly equal numbers of men and women—a different world from a mere 30 years ago, when a women could be rejected on the basis of marital status rather than MCATs, when a small band of five women among 245 men was the norm, and when hazing the women med students was fair game for both faculty and students.

The current social experiment in equity for men and women is not easy. Both men and women occasionally become upset by the consequences, complaining about "pushy women" or "the good ol' boys." Women in particular become frustrated that things are not moving fast enough.or that they have simply taken on two jobs, keeping the one at home while also having one outside the home.

Given the difficulties of this experiment, as well as its importance, it is crucial that studies of "gender differences" employ sound science that is interpreted judiciously and impartially. Poor scientific evidence has been used for years to justify denying equal opportunities to women. Why waste a medical school slot on a female who is not good at math and has no aptitude for science? Why place a woman in a leadership role when she is the victim of hormones that interfere with her judgment? Studies of gender differences can be a double-edged sword: they may help with the understanding of genuine differences in vulnerability or disease prevalence in both sexes, or they may be used to promote unfair social policy.

As clinicians and scientists, what we need are not biased opinions and studies shaped by archaic preconceived notions but rather open minds and studies conceived without bias that report their findings accurately and objectively. Studies that examine gender differences in disease prevalence, brain biology, or mental abilities should be based on careful science, carefully interpreted. The studies in this issue reflect that balanced style.