We find that the gender ratio of peers in high school significantly affected the choice of major. A larger share of same-sex peers increases the probability of choosing majors associated to high earning jobs (Economics/Business, Medicine, Engineering). For women we also find that a large percentage of female high school classmates increases their long run performance in college and their earnings.
From the new paper "The Long Run Effects of High-School Class Gender Composition" by Anelli & Peri (January 2013).
The authors conclude
We speculate that this effect could be the result of higher confidence and higher willingness to compete for women who are in mainly female environments, as it has been pointed out by previous experimental literature. The interesting novelty of our result is that we find a similar effect at work for men and that we find long-lasting effects on academic performance and labor market outcomes from being exposed to a large same sex share of peers in high school.And
Indeed female high school students exogenously assigned to classes with high female shares are on average 5.7 percentage points more likely that other women to enroll in a high earning major with respect to a baseline probability of 20.8 percent (while higher male shares increase male students’ probability to enrol high earning majors by the same amount). Women are 4.4 percentage point more likely to graduate from a high earning major. They take one month less to graduate from those majors relative to other women and they earn similar final grades. Finally they have a earning premium of 20%, relative to other women five to twenty years after high school graduation.