It is thought that if parents give birth to sons or daughters, it is simply a chance. But not exactly! The normal sex ratio at birth is 105 men and 100 girls. But the relationship depends on the circumstances and the differences between the families. There are other factors that influence the sex of a offspring. One of the most embraced principles of evolutionary biology is the Trivers-Willard hypothesis, which states that powerful parents with a high social status give birth to more boys, while poor parents give birth to more girls. This is because children generally inherit the wealth and social status of their parents.

Throughout history, sons from wealthy families who would one day become like themselves, were expected to have large numbers of wives, girlfriends, and more children, while their equally wealthy sisters could only have one. limited number of children. In this way, natural selection forces parents to bias the sex ratio at conception, depending on the economic situation: more boys if they are rich and more girls if they are poor. This hypothesis is well known and documented around the globe.

US presidents, vice presidents and cabinet secretaries have had more boys than girls. According to church files in the 17th and 18th centuries, wealthy landowners in Leezen in Germany had more sons than daughters, while farmers and traders without any property had more daughters than sons. In a survey in 46 countries, the question "if you had a baby, what gender would you like" the rich answered "boy" while the poorest answered "girl". The Trivers-Willard hypothesis goes further than the status and wealth of a family; if parents had any qualities that they could pass on to their children and that quality was more appropriate for boys than for girls, then they gave birth to more sons.

Physical appearance has a greater impact on reproductive success in females than in males. The general hypothesis predicts that handsome parents would give birth to more girls than boys. Americans who have been labeled "extremely attractive" have a 56% chance of giving birth to a daughter as their first child, compared to 48% for the rest of the population.