What percentage of our ancestors were men?

From Is There Anything Good About Men? by Roy F. Baumeister

Here’s a riddle for you: What percentage of our ancestors were men?

It’s not a trick question, and the answer isn’t 50%. True, perhaps half of all the humans who were ever born were male, but that’s not the question. We’re not asking about everyone who ever lived. We’re asking about everyone who lived and who also has a descendant alive in the world today.

[snip]

Let’s return now to the question of what percentage of our ancestors were women. Yes, each baby has one mother and one father, so each baby’s parents were 50% male. But some of those parents had multiple children, and not necessarily always with the same partner. Every baby’s parents are 50% male, but you can’t extrapolate from that to conclude that today’s human population has an ancestry that is 50% male.

The correct answer has recently begun to emerge from DNA studies, notably those by Jason Wilder and his colleagues. They concluded that among the ancestors of today’s human population, women outnumbered men about two to one.

Two to one!

In percentage terms, then, humanity’s ancestors were about 67% female and 33% male.

To illustrate how this could be possible, imagine a desert island at the start of time with just four people: Jack, Jim, Sally, and Sonya. Thus the population is 50% female. Let’s assume Jack is rich and handsome, while Jim is poor and unattractive, so Jack marries both Sally and Sonya. Thus, Jack and Sally’s baby, Doug, has ancestors who are 50% female (i.e., Jack and Sally). The same can be said for Jack and Sonya’s “baby, Lucy. But if you take Doug and Lucy together, their combined ancestors are 67% female (because their total ancestors are Jack, Sally, and Sonya).

Most people are surprised to hear that humankind today had twice as many female ancestors as male ones, because they thought it would be closer to 50:50. When experts hear about this, they are surprised too, but often for the opposite reason: They thought the imbalance would be even more severe. That is, they thought it would be maybe 75% to 85% female. Probably it was more severe through much of history, and especially prehistory. In many animal species, close to 90% of the females but only 20% of the males reproduce. The way the human population has ballooned in recent centuries means that most people who ever lived are either alive today or were alive recently, and in modern times the rule of monogamy has spread over large parts of the globe. In past eras, when polygamy (one husband, multiple wives) was the norm, the reproductive imbalance would have been even more severe. Hence whatever conclusions we draw about the differences between men and women based on the two-to-one ancestor “ancestor difference are probably understatements. If we had done the research even just a few centuries ago, the ratio might have been three female ancestors to every male one, or four to one.

What does it mean that we are descended from twice as many women as men? It can be explained like this. Of all the people who ever reached adulthood, maybe 80% of the women but only 40% of the men reproduced. Or perhaps the numbers were 60% versus 30%. But one way or another, a woman’s odds of having a line of descendants down to the present were double those of a man.

Also, crucially, the majority outcome is different—the most common outcome of normal life. Most women who ever lived to adulthood probably had at least one baby and in fact have a descendant alive today. Most men did not. Most men who ever lived, like all the wild horses that did not ascend to the alpha male’s top spot, left behind no genetic traces of themselves.

That’s a stunning difference. Of all humans ever born, most women became mothers, but most men did not become fathers. You wouldn’t realize this by walking through an American suburb today with its tidy couples. But it is an important fact. I consider it the single most under-appreciated fact about the differences between men and women.