How birth order and the sex of siblings affect personality are some of the most debated topics in psychology. A new study suggests that boys who have older sisters are less competitive than those who don't.Researchers asked groups of secondary school pupils and university students to choose between two types of game.The first was a "piece-rate payment system" in which participants build up points, working individually. In the second "tournament system" a larger reward was available but only if the schoolboys beat others in their group.Overall, the schoolboys were more likely to pick the competitive option (61%) than girls.But when only boys with older sisters were considered, that number dropped to only 38%.When it came to the university students, men were more competitive than women, but men with an older sister were 21% less likely to chose the tournament option than those without. ..