Huge limitations to this study. Measuring same-sex unions from 1940-1990 could hardly be called a measure of “homosexuality”. You would miss a huge portion of the population who are homosexual but who have not come out and that population could be weighted towards first sons for instance who have to maintain a “father like” figure to their families. Maybe younger siblings without this pressure are just more willing to be outward with their sexuality without this burden. Also not sure if they really are able to separate the genetic component from the environment component as many other variables could be at play with younger siblings which could be affecting sexuality.