Female twins who shared the womb with a brother are better at visualizing shapes being rotated than those who shared the womb with a sister, according to a study in Psychological Science. Sex differences in mental rotation tasks–in which participants try matching rotated versions of 3-D block figures–have been linked to testosterone levels, with males outperforming females from an early age. Previous studies have reported that female twins from opposite-sex pairs are exposed to higher levels of testosterone in the womb than those from same-sex pairs. That degree of testosterone exposure appears to masculinize certain physiological features, such as finger-length ratios. In the present study, 804 twins, the average age of which was 22 years old, performed a mental rotation test in which they matched figures that were identical but rotated. Out of a maximum score of 24, females with a twin sister scored 9.01 on average, while females with a twin brother scored 10.26–a statistically significant difference after the researchers factored in age, birthweight and other variables. Male twins from same-sex pairs scored 12.87, while those from opposite-sex pairs averaged 13.74, but the difference between the two groups wasn’t statistically significant.
Caveat: Environmental differences between same-sex and opposite-sex twins might have influenced rotation test scores.