My great-grandmother sold her youngest son in order to buy a bride for my grandfather.

I asked why my grandfather didn’t shop around, maybe try to find a cheaper wife. Apparently that’s not how human mating behavior works.

In most historical societies, a bride’s parents require payment because the woman marries into the husband’s family [1]. The offspring bear the paternal family name, till the paternal land, and defend the paternal tribe. Brides-to-be are valuable for their ability to spawn future utility.

Under normal conditions, the gender ratio is roughly 1:1. Oftentimes, such as during war, supply exceeds demand due to all the men who die fighting. Why don’t periods of excess supply drive bride prices down to zer0?

Blame polygamy. Human reproduction is an embarrassingly parallel problem. A male that wants ten offspring would get them quicker by fertilizing ten wives at once, instead of waiting 90 months for a single wife. In fact, she’d probably die by about 6 or 7.

Thus emerged the practice of wealthy men taking multiple wives. Bride price levels are maintained because many women would rather be an emperor’s umpteenth concubine than the monogamous partner of a peasant.

The sale and transfer of women leads to a tendency to treat wives like property. To mitigate this condition, families marry their daughters off with a dowry. This is a collection of assets that belong to the bride, which she can later sell and use for protection if her husband turns out to be a shit.

To address the initial question:

Poor families are better off bearing daughters, whom they can sell as brides.

Wealthy families should have a boy, and buy him lots of brides so that he can engender an army with which to defend the family wealth.

If you don’t live in an archaic transactional-marriage society, you should love all your offspring equally.

Interesting Notes:

Evolutionary biologists have theorized that sex ratios are influenced by economic conditions, where female offspring are more likely when resources are scarce [2]. Evidence of this claim in human populations has been statistically questionable.

Tibet is one of the few places I know of where women have multiple husbands. This is possibly due to land laws that prohibit property fragmentation.

References:
1. Schlegel, A. and Eloul, R. (1988), Marriage Transactions: Labor, Property, Status. American Anthropologist, 90: 291–309. doi:10.1525/aa.1988.90.2.02a00030
2. R.Trivers, D.Willard. Natural Selection of Parental Ability to Vary the Sex Ratio of Offspring, Science. 1973 Jan 5;179(4068):90-2.