It’s a scientific fact that high social status is attractive to women. Fertile women prefer more dominant men and the lucky few who achieve money or influence tend to marry younger, more often and have more extra-marital affairs than their peers. If your son is a success, it could be a big evolutionary win. But if he isn’t, he may find himself unable to find a partner at all.

“Because women tend to marry ‘up’ the socioeconomic scale – it’s as if women trade in their good looks to some degree – if you’re a man at the bottom of the scale you’re going to struggle to find someone to marry,” says Trivers. Females, by contrast, tend not to face such stiff competition; they have a higher chance of securing a mate and producing some offspring, even if they will never result in as many descendants as a son.

That may sound a little bit sexist, but Trivers argues that it arises from the fact that a female invests more in the young, compared to a male who can just have sex and leave the childrearing to the mother. Consider the fact that the most prolific mother in world history was the nameless first wife of a peasant from Shuya, Russia, who lived from 1707 to 1782. In total, local records reveal that she gave birth to 69 children, which is nothing compared to the warrior Genghis Khan, who fathered between 1,000 and 2,000 children before he died in 1227. Today, he’s thought to have around 16 million great, great, ... great grandsons. And Khan is not the only one; a recent DNA analysis revealed at least 10 men from history who have left a legacy comparable with Khan’s, including a Chinese ruler who died in 1582, and the originator of the medieval Uí Néill dynasty in Ireland.

In many animals – red deer, elephant seals, gorillas – the stakes are even higher. Successful males may have harems of hundreds of females, while low-ranking or weak males may never reproduce or die trying.

Then there’s the issue of resources. Because they tend to be larger, sons require a lot more food than daughters and in many societies they’ll require more education and money. To produce a son capable of becoming a dominant, high-status male, parents will need to make a big investment.