Evolution Explains Your Grandma

My great-grandmother’s ninety-something birthday was a little while ago, and while thinking about how awesome she is (and how big the slices of pie she always gives me are) I thought it’d be a good time to write about grandmas in general. There’s a rhetorical question posed to first-year evolution students everywhere: what’s the point of being a grandma? It’s meant to shock students into thinking a bit more abstractly about genetics, but invariably some jackass tries to answer with something like "Well, if grandmas can’t have kids, then evolution says they’re pointless."

That’s obviously and unequivocally false. Grandmothers are wonderful. They give you presents and love and hard candy and hard advice. Their job is to spoil you rotten, and even from a biological viewpoint there’s an enormous amount of value in that. But evolutionarily, what are grandmas getting out of the deal? Even more weirdly, why do women experience menopause?

We’ve talked about evolution at its most basic mechanism: traits that help make more babies will have more copies of themselves than less successful traits in the next generation, and eventually the less successful traits get rarer and rarer until they disappear. From a personal standpoint, your goal is to make as many successful copies of yourself as possible, which in the human case means lots of feeding and nurturing. We don’t act like salmon, having hundreds of kids in the hope that a few survive. Instead, we tend to focus a lot of attention on just a few kids because the modern human world takes a lot more training to navigate.

But why should grandmas waste their time spoiling their grandkids rather than focusing on making more kids of their own? Well, in theoretical terms, it’s a matter of probability.

Unlike amoebae that self-replicate, our kids carry half of our genes and half of the genes of whatever bum we convinced to have kids with us. To calculate your biological fitness, or the probability of your traits being passed on to another generation by your kids, you multiply the percentage of your genes in your kid (always 50 percent) by the odds of your kid living long enough to have kids of their own (partly dependent on how good of a parent you are and how great your genes are).

Good grandparents will help increase your fitness, perhaps by babysitting a toddler that otherwise might eat drain cleaner if left to her own devices or convincing little Timmy not to shoot heroin. That’s great for you and your kids, but what about Granny?

Grandparents have a vested interest in their grandkids because, probability-wise, those kids are 25 percent of themselves. So by helping raise grandkids you help ensure that at least some of your traits carry on, which residually adds to your fitness. Sure, if Granny focused on having more of her own kids instead, she’d be potentially creating higher-percentage copies of herself. But if those kids die—perhaps because childbirth is more risky for older mothers—her fitness increases by zero.

That’s the convoluted reason why evolutionary scientists propose menopause exists. They posit that woman are reach a point where having kids has far less of a potential fitness gain than helping raise grandchildren does.

Yet only women fully lose their reproductive capability after menopause. Why? Evolutionarily, a woman’s bodily reproductive efforts are more energy-intensive than a man’s. Creating an egg and all of the supporting tissues once a month takes a lot more energy than sperm. More importantly, pregnancy and child birth are a hell of a lot more taxing on a woman than a man.

That may be the root cause of menopause: there comes a point where there risk/reward ratio for having kids becomes so unfavorable that menopausal traits were selected for women because it allows them to save energy better spent on grandmotherly duties. It’s like an evolutionary grandmother switch.

We do see similar things in men, however. There is debate about whether male menopause affects all men, but the symptoms are common: an age where testosterone levels, sperm counts and sex drive all decline rapidly. Unlike women, men don’t experience a fundamental change to their reproductive organs, but then again it’s less energy-intensive for men to stay at least partially reproductively capable month-to-month.

So is the point to say that your grandparents have a selfish interest in caring for you? No more than your parents do, which doesn’t matter either way. I mean, there’s an evolutionary component to them wanting to see you succeed, but that’s not everything. As anyone with stepparents (or, in my case, some step-grandparents) knows, parental love isn’t simply a selfish, fitness-obsessed phenomenon. Just remember this: the next time your grandma starts shoving fruitcake and ugly-yet-warm sweaters in your face, it’s because she’s got a stake in you.

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