The recent and thought-provoking Slate Star Codex sequence on cultural evolution has led me to think about traditional sexual ethics, and the fact that it is literally impossible to do them in the modern day.

There are three large changes that have occurred in the past few hundred years, which affect sexual ethics. The first and most obvious is the invention of birth control, which permits people to separate (penis-in-vagina) sex and babies.

It is easy to overstate the importance of birth control. Many effective methods of birth control, such as homosexuality and outercourse, were known since the Paleolithic. The Oneida Community reportedly had a typical use pregnancy rate of 0.5%, more effective than modern birth control pills, with male continence; this is a method known since Biblical times. (Of course, the Oneida Community may have had particularly motivated users, and widespread use may have been less effective.)

Nevertheless, giving people more birth control methods with fewer side effects and no chance of not using them in the heat of the moment likely changes many things about sexual ethics.

Second, children are now a net financial drain on their families. In the developed world, children are always a financial cost for 18 years, and often for longer than twenty; they rarely pay their parents back. However, historically and in the developing world, children often began making a financial contribution as young as seven. It is difficult to estimate how many children are/were involved in child labor and how large their contributions to the household were. However, even today, in large families, teenagers who are not sent to school can often pay for themselves through chores and taking care of younger siblings; there is no reason to believe this was not true in the past. (I am interested in more detailed data and am happy to edit this section with more.)

Finally, and most importantly, child mortality.

Our World In Data provides some interesting graphics about child mortality in the past two hundred years. In summary: in 1800, while there is little data, the best estimates suggest that about 40% of children died before age five. In 2019, in rich countries, less than one percent of children die before age five.

Forty percent is a lot of children. Consider a fairly ordinary traditional Catholic family of five children: in 1800, they would only have had three. A family of ten would, in 1800, only have six children. Even the Duggars’ nineteen children would only have been eleven.

But high child mortality rates affect more than family size. That forty percent isn’t evenly distributed among families; some may bury seventy percent of their children, perhaps because of a series of epidemics or a bad crop year. If all you care about is one of your children surviving to take care of you in your old age, and the mortality rate is less than one percent, you have one child. However, if the mortality rate is forty percent and unevenly distributed, you may have to have many more than two kids to have a chance one of them survives to adulthood.

(The evidence is suggestive that decreasing child mortality tends to decrease fertility, in part for this reason.)

What this means is that practicing truly traditional sexual ethics is literally impossible.

You could stop using birth control, and people do. (Catholics use natural family planning, but natural family planning is itself a fairly recent invention. You could, fortunately, do extended breastfeeding for a break in between pregnancies.) In theory, it is required that you educate your children. In practice, you can homestead in a state that doesn’t check up on homeschoolers much and put your children to work farming or watching their younger siblings as soon as they’re able. It wouldn’t be doing right by your kids– it turns out some knowledge of writing and math and history and science is useful for being alive in the 21st century– but you could do it.

But child mortality is a bitch. “Not using birth control” is unpopular, and “educationally neglect your children in order to live on a homestead” is unpopular, but “forty percent of your children die” is more unpopular than either of those. There exist some religions that don’t use modern medicine, but you’re never going to get particularly widespread uptake.

But even if you are a Christian Scientist homeschooler who doesn’t use birth control, you’re still not going to get to the environment that traditional sexual ethics evolved for. Many of that forty percent died in epidemics, and most of the diseases they died of have been eradicated in the United States due to vaccines. You are never, ever going to have three of your children die of smallpox in a single month.

These changes are generally agreed upon to be good things among both sexual liberals and sexual conservatives. No one wants forty percent of their children to die. Child labor is generally unpopular. While some social conservatives disapprove of birth control, most social conservatives do not.

But it means that you can’t make the argument “the sexual ethics of 1800 are good because they are traditional and worked for hundreds of years.” Our situation is very very different from the situation in 1800. Children are financial drains instead of investments; children are almost certainly not going to die; it is possible to separate PIV from reproduction with a good deal of reliability.

This is not, of course, to say that the traditional sexual ethics of 1800 are incorrect for modern humans. It may well be that we would all be happiest if divorce and sodomy were illegal, no one used birth control, having sex before marriage if you’re a woman made you a fallen woman, and men are technically not supposed to have sex outside marriage but in practice seeing a prostitute is a common vice among urbanites. But this proposition– in the current situation– has at best a few decades of track record. It cannot take advantage of the argument from tradition, any more than can the proposition that we would all be happiest if gay marriage were legal, divorce were unstigmatized, many people were poly, and birth control is the default.

We knocked over the Chesterton’s Fence, because Chesterton’s Fence was driven through the heart of millions of children and subjected them to a horrible painful death. Now we have to figure out sexual ethics in a fencefree world. Chesterton’s Fence does not apply.