Slate’s archives are full of fascinating stories. We’re republishing this article because it remains a reader favorite. It was originally published Jan. 10, 2013.
When the Explainer asked you to vote on a favorite unanswered question of 2012, the majority opted for a rather lascivious query regarding why rich ladies sunbathe topless, and the Explainer has duly delivered your pound of flesh. But in perusing the runners-up, another question so intrigued the Explainer that he could not resist answering it as well: When and how did humankind figure out that sex is what causes babies? It’s not exactly the most obvious correlation: Sex doesn’t always lead to babies, and there’s a long lead time between the act and the consequences—weeks before there are even symptoms, usually. So roughly where do we think we were as a species when it clicked?
Basically, since the beginning. While anthropologists and evolutionary biologists can’t be precise, all available evidence suggests that humans have understood that there is some relationship between copulation and childbirth since Homo sapiens first exhibited greater cognitive development, sometime between the emergence of our species 200,000 years ago and the elaboration of human culture probably about 50,000 years ago. Material evidence for this knowledge is thin, but one plaque from the Çatalhöyük archaeological site seems to demonstrate a Neolithic understanding, with two figures embracing on one side and a mother and child depicted on the other. A firmer conclusion can be drawn from the fact that, though explanations for conception vary wildly across contemporary cultural groups, everyone acknowledges at least a partial link between sex and babies.
As for how humans attained what biological anthropologist Holly Dunsworth calls “reproductive consciousness,” that part is murkier. Most likely, we got the gist from observing animal reproduction cycles and generally noting that women who do not sleep with men do not get pregnant. But that doesn’t mean that early peoples—or for that matter, modern people—thought or think of the process in the utilitarian, sperm-meets-egg way that the scientifically literate do now.
Around the turn of the 20th century, anthropologists working in places such as Australia and New Guinea reported that their subjects did not recognize a connection between sex and children. However, subsequent research has shown these biased reports to be only half-true at best. For example, Bronislaw Malinowski claimed in 1927 that, for Trobriand Islanders, the father played no role in producing a child. But later anthropologists studying the same group learned that semen was believed to be necessary for the “coagulation” of menstrual blood, the stoppage of which was thought to eventually form the fetus.
Even though the Trobriand Islanders’ traditional explanations of conception seem quaint or strange, they do on some level recognize the tie between sex and childbirth. And of course, before we Westerners get to feeling all superior, it must be said that our notions of conception are not wholly consistent or rational either. (The number of unplanned pregnancies in the United States reveals as much.) As women’s studies scholar Cynthia Eller points out, while “other events may also be necessary—such as the entrance of a spirit child through the top of the head (in the case of the Triobriand Islanders), or the entrance of a soul into a fertilized egg (in the case of Roman Catholics) … it is simply not believed that women bear children without any male participation whatsoever.”
If we humans have essentially always kind of understood that the deed leads to the delivery room, did that knowledge have any consequences on our evolution as a society? Holly Dunsworth argues that, of the entire animal world, “reproductive consciousness” is unique to humans. That special knowledge may help explain both the evolution of our taboos around sex and our ability to bend nature’s procreative capacities to our favor in everything from dog-breeding to family planning.
Explainer thanks Holly Dunsworth of the University of Rhode Island, Cynthia Eller of Montclair State University, Helen Fisher of Rutgers University, and Wenda Trevathan of New Mexico State University.