gray and WM density and their relationships to estradiol and testosterone levels (Peper et al., 2009). Males (n=37; age 11.6 +- 1.0 yrs) had overall larger brain volumes than females (n=41, age=12.2 +-1.2yrs) after correction for age differences, although no difference was seen in the ratio of gray to WM between the sexes. The largest regional sex differences were in the putamen, insula, and amygdala, all larger in males. Total GM volumes correlated negatively with estradiol levels in females and positively with testosterone levels in males. A limitation of this study was that the female twin pairs were significantly older than the males. While a factor to account for age was included in all analyses, the authors note that it is possible that males were on the upward portion of their developmental trajectory in brain volume while females were on the downward trajectory, making exact comparisons difficult.
