8 Is Enough: The Limits to Human Reproduction
Researchers made face-shape tweaks to images of baby faces using a computer program. The tweaks resulted in five images of the same baby, such as this six-month-old, along a cuteness continuum from 50 percent (meaning 50 percent cuter than average) to minus 50 percent, or less cute than average.
Credit: The Perception Laboratory, School of Psychology, University of St. Andrews.

Editor's Note: The mother of octuplets previously had six children, it was revealed today.

Eight kids at once. The mind boggles. The mind is also pretty creeped out by the thought of one tiny baby after another coming out of a woman as if she were a mouse.

It's great those octuplets are here and healthy, but really, humans aren't designed to have litters.

It's basic energetics. Every individual has only so much energy. Some energy is spent staying alive — that is, finding food and not being somebody else's food — and what's left over can be spent on reproduction. In other words, there are limits to reproduction.

Of course, the various slices of that reproductive energy pie also vary between males and female of all species. Males don't gestate or lactate so they pass on the most genes by flitting from female to female making as many babies as they can, and then walking away. The female reproductive pie is much more complex. There are costs to pregnancy, lactation for mammals, and then whatever else is needed to bring a kid up to sexual maturity so they can pass on genes as well.

But there are all sorts of ways, from an evolutionary point of view, for females of a species to distribute that energy and bring up babies successfully. She might have as many babies as she can in one shot, litters that is, and have them as often as possible. For that kind of female, reproduction is an assembly line of cheap production per kid. Or a female might opt for the other end of the scale and make one baby at a time and wait for a very long time to see if that one investment pays off.

Obviously, humans are on slow side of the baby production continuum. Evolution has selected for this path because there are features of our species that require great investment by mothers. Human infants might have big brains compared to other mammals, but they need to get even bigger once outside the womb. And so human infants are actually born neurologically unfinished. They can’t cling, sit up, feed themselves, or run from predators. And so the very nature of what it takes to be an adult human puts constraints on how many children a mother can have at a time.

Take a look at a naked woman and see for yourself how many babies a woman is designed to care for — two, at most.

In fact our babies need so much that human fathers, too, have been selected to stop fooling around and to stay at home with one female if they want to see their genes go forward. The evolution of the human family is not about men and women deciding to make a commitment; it's really about the dependency of our children.

And we are so used to this system that we can't help but stare in awe at twins and scream in shock at octuplets. It doesn't seem right to have that many kids at once because it isn't right in the evolutionary sense. That family will surely have help beyond the mother and father, and they will all probably have a great time, but chances are it will be a very long time before that mother reproduces again.

Meredith F. Small is an anthropologist at Cornell University. She is also the author of "Our Babies, Ourselves; How Biology and Culture Shape the Way We Parent" (link) and "The Culture of Our Discontent; Beyond the Medical Model of Mental Illness" (link).