As scientists learn more and more about genetics, humans become increasingly nervous about our ability to basically engineer a child to be exactly the way we want (which is a quiet girl, duh). But it turns out that our mammal brethren have actually been picking the sex of their babies in the animal kingdom for a long time. Lions, tigers and bears: always so ahead of the curve.
Researchers at the San Diego Zoo (go there, it's rad) used their access to three generations of different mammals to study if, of the ones that predominantly had offspring of one sex, those offspring went on to have the strongest children who could then go on to have many strong kids of their own:
...the number of offspring a male produces is often limited by how many females he can mate with, while a female is limited by how many offspring she can physiologically produce. This generates a tendency for males to vary more in first-generation success than females. Thus male offspring are a high-risk-high-reward bet for potential grandparents in the genetic lottery; while females are a safe, hedged bet. However, just like in insects, if a grandparent ‘knows’ that a male offspring is a low-risk-high-reward bet, then they can beat the house, and hit a jackpot (in terms of grandchildren produced).
That's always what I say when I'm deciding which child to have. Those males, so high-risk-high-reward — but the FEMALES, yeesh, such a safe, hedged bet. The Washington Post notes:
In mammals, how gender selection works remains a mystery. Early ideas regarding male influence, such as alpha males somehow developing a higher percentage of male-oriented sperm, were largely debunked.
“Which means females are really the ones controlling the situation,” said Garner, who calls the strategy “sneaky Machiavellian girl power.”
Some researchers believe that humans do this too, which means that at whatever baby shower I have to go to next, I will totally accuse the parents of manipulating the sex of their baby, causing a huge scene which culminates in me being left alone at the mini-sandwich table, perfectly happy and content.