Academic performance of opposite-sex and same-sex twins in adolescence: A Danish national cohort study

Abstract

Testosterone is an important hormone in the sexual differentiation of the brain, contributing to differences in cognitive abilities between males and females. For instance, studies in clinical populations such as females with congenital adrenal hyperplasia (CAH) who are exposed to high levels of androgens in utero support arguments for prenatal testosterone effects on characteristics such as visuospatial cognition and behaviour. The comparison of opposite-sex (OS) and same-sex (SS) twin pairs can be used to help establish the role of prenatal testosterone. However, although some twin studies confirm a masculinizing effect of a male co-twin regarding for instance perception and cognition it remains unclear whether intra-uterine hormone transfer exists in humans. Our aim was to test the potential influences of testosterone on academic performance in OS twins. We compared ninth-grade test scores and teacher ratings of OS (n = 1812) and SS (n = 4054) twins as well as of twins and singletons (n = 13,900) in mathematics, physics/chemistry, Danish, and English. We found that males had significantly higher test scores in mathematics than females (.06–.15 SD), whereas females performed better in Danish (.33–.49 SD), English (.20 SD), and neatness (.45–.64 SD). However, we did not find that OS females performed better in mathematics than SS and singleton females, nor did they perform worse either in Danish or English. Scores for OS and SS males were similar in all topics. In conclusion, this study did not provide evidence for a masculinization of female twins with male co-twins with regard to academic performance in adolescence.

Sex differences in mathematical achievement: Grades, national test, and self-confidence

“…There is a theory that during the prenatal period, having an opposite-sex co-twin can change the level of prenatal testosterone, resulting in differentiated brain structure and masculinization of girls (Tapp et al, 2011;Ahrenfeldt et al, 2015). This means that girls from opposite-sex pairs are more likely to pursue activities linked to the development of spatial abilities (Berenbaum et al, 2012;Constantinescu & Hines, 2012) and have fewer differences from boys in mathematical ability and mathematical achievement (as found in our study).…”

Early-life mortality risks in opposite-sex and same-sex twins: a Danish cohort study of the twin testosterone transfer hypothesis

“…There are no studies of amniotic fluid testosterone levels from OS and SS twin pairs [35], and studies comparing OS with SS females have shown inconclusive results with the most consistent evidence to support the TTT hypothesis coming from studies of perception and cognition [11]. Several studies that have investigated co-twin effects in males have failed to identify differences between OS and SS male twins, for example, with regard to tooth size [36], academic performance [37], and cancer [38]. However, animal studies find that males exhibit reduced immune responses and increased intensity and prevalence of infections compared with females [4], and in humans, there is evidence that girl infants have lower mortality from infections [6] and respiratory conditions [7] compared with boys, which may reflect the immunosuppressive effects of testosterone [4,5].…”

Sex Differences in Human and Animal Toxicology

“…The evidence is sufficient to warrant further investigation in humans (Tapp et al . 2011), however, Ahrenfeld et al 's (2015) study of 9 th graders found no difference in school performance on either visuospatial or language skills between girls from FF or MF twins. Since twin studies are popular, the paucity of evidence indicative of TTT in human twins suggests that it is minor and/or rare.…”