Many people still believe on ideological grounds that men and women are neurologically identical, or that the differences can have, must have, no bearing whatsoever in expressed behaviour.
Well, it is statistical, individuals vary, there's no such thing as a "boy brain" or a "girl brain", just brains whose structures match typically male or typically female stereotypes to a greater or lesser extent... but close enough. And the two patterns converge it seems, at least as regards the cerebral cortex. Not the base of the brain though, the seat of emotional response, body map, and gender identity. If anything, there the differences grow more marked.
From the Wall Street Journal, As Little Girls and Boys Grow, They Think Alike
The NIMH, as part of a 20-year-old brain-mapping project, has been doing MRI scans of young people's brains, age 9 to 22. By measuring the thickness of the brain's cortex and how it changes over time, scientists have found that boys' and girls' brains, on average, differ significantly at age 9. But by the time the participants reached age 22, the brains of the two sexes grew more alike in many areas critical for learning. In general, most parts of people's brains are fully developed by the age of 25 to 30. The NIMH study, which involved 284 people, was published last year in the journal Proceedings of the National Academy of Sciences.One problem: the metric used is pretty crude. Just thickness of cerebral cortex. And we know that that is affected by the hormonal environment.
Another finding: Young girls' brains tend to mature faster in the front part, which is responsible, among other things, for language learning and controlling aggression and impulsivity. For boys, the fastest development is in the back of the brain, which performs visual-spatial tasks at which males tend to excel such as geometry and puzzle-solving.
Changing your sex changes your brain: influences of testosterone and estrogen on adult human brain structure by Pol et al, Europ Jnl Endocrinology, Vol 155, suppl_1, S107-S114 2006
Objective: Sex hormones are not only involved in the formation of reproductive organs, but also induce sexually-dimorphic brain development and organization. Cross-sex hormone administration to transsexuals provides a unique possibility to study the effects of sex steroids on brain morphology in young adulthood.
Methods: Magnetic resonance brain images were made prior to, and during, cross-sex hormone treatment to study the influence of anti-androgen + estrogen treatment on brain morphology in eight young adult male-to-female transsexual human subjects and of androgen treatment in six female-to-male transsexuals.
Results: Compared with controls, anti-androgen + estrogen treatment decreased brain volumes of male-to-female subjects towards female proportions, while androgen treatment in female-to-male subjects increased total brain and hypothalamus volumes towards male proportions.
Conclusions: The findings suggest that, throughout life, gonadal hormones remain essential for maintaining aspects of sex-specific differences in the human brain.
Nonetheless, the differences are marked before age 9.
Gender DevelopmentAnd not so surprisingly, it appears that transsexual childrens' behaviour is more like that of the sex opposite to their appearance. The same anomaly is present in some Intersex conditions too, notably CAH in girls. Furthermore, it appears that a significant fraction of those who will grow-up with a non-standard sexual orientation - homosexual or bisexual - show the same pattern, though most do not.
Some typical milestones and when boys and girls tend to hit them:
At birth: Girls are a few weeks more mature neurologically and have more advanced hearing. Boys on average weigh half a pound more.
First words: Girls typically utter their first word at 11 or 12 months, one month ahead of boys.
Vocabulary: At 18 months, girls on average know 86.8 words, more than double boys' 41.8 words. By 30 months, boys' and girls' language skills have converged, at about 500 words.
Walking: Caucasian girls and boys tend to walk around 12 months. African-Americans walk sooner, at nine to 10 months.
Potty training: Girls are fully trained by 36 months, according to one study. Boys took a bit longer, training by 38 months.
Onset of puberty: For girls, the process can start at age 9 to 10. For boys, it's closer to 11 to 12.
Source: WSJ research