Mars-Venus sex differences appear to be as mythical as the Man in the Moon. A 2005 analysis of 46 meta-analyses that were conducted during the last two decades of the 20th century underscores that men and women are basically alike in terms of personality, cognitive ability and leadership. Psychologist Janet Shibley Hyde, PhD, of the University of Wisconsin in Madison, discovered that males and females from childhood to adulthood are more alike than different on most psychological variables, resulting in what she calls a gender similarities hypothesis. Using meta-analytical techniques that revolutionized the study of gender differences starting in the 1980s, she analyzed how prior research assessed the impact of gender on many psychological traits and abilities, including cognitive abilities, verbal and nonverbal communication, aggression, leadership, self-esteem, moral reasoning and motor behaviors.

Hyde observed that across the dozens of studies, consistent with the gender similarities hypothesis, gender differences had either no or a very small effect on most of the psychological variables examined. Only a few main differences appeared: Compared with women, men could throw farther, were more physically aggressive, masturbated more, and held more positive attitudes about sex in uncommitted relationships.

Furthermore, Hyde found that gender differences seem to depend on the context in which they were measured. In studies designed to eliminate gender norms, researchers demonstrated that gender roles and social context strongly determined a person's actions. For example, after participants in one experiment were told that they would not be identified as male or female, nor did they wear any identification, none conformed to stereotypes about their sex when given the chance to be aggressive. In fact, they did the opposite of what would be expected - women were more aggressive and men were more passive.

Finally, Hyde's 2005 report looked into the developmental course of possible gender differences - how any apparent gap may open or close over time. The analysis presented evidence that gender differences fluctuate with age, growing smaller or larger at different times in the life span. This fluctuation indicates again that any differences are not stable.