The Grandmother Hypothesis

So there seems to be mild to moderate interest in me rattling off about evolutionary psychology. I'm going to make an effort to separate my own musings from what I've learned- but in some cases, the whole thing's gotten mixed up. Thus, citations.

The grandmother hypothesis is a theory that addresses the conundrum of female longevity. Males can keep producing sperm into their second century, but a female giving birth after 50 is rare. Old Darwinism holds that if an organism can no longer reproduce, there isn't really any point in continuing to survive- so why do women live years and years past menopause- and on average, much longer than men?

One very popular theory is that grandmothers- and to some measure both sets of grandparents- increase the chances that children will survive. Another pair of eyes can keep someone from toddling into a sink hole, another pair of arms can carry around a miserably sick baby for a few essential hours, another pair of eyes can find some tasty treats, and another mind can remember and teach all sorts of skills.

There's a corollary to this theory; post menopausal women are frikkin' juggernauts. Since there's no longer any need to keep the body able to reproduce, they can drive themselves so very very hard and sacrifice everything for the good of their offspring. Recall how pTerry said there were entire civilizations based on the lifting power of little old ladies in black dresses? This explains why the only sport where women have an advantage over men is ultramarathons, where ladies in their fifties hold many of the crazier records. This explains why your mother is so much more productive than you are.

Everyone loves this theory, both because it makes intuitive sense and because it emphasizes those traits we admire in ourselves. Unfortunately, evidence is thin. It's pretty difficult to separate children who survived because grandma was around from children who survived because they had awesome genes, as did grandma- the proof that they are awesome is that she was still around.

An early experiment (not mentioned in the wiki article, but in my evolution textbook) looked at records of Canadian frontier families during the 19th and early 20th century. This is a wonderful combination of excellent record keeping with a horrible environment for children. As an illustration let's use the Little House books which are set immediately to the south; in the Dakotas, Minnesota, Kansas, and Wisconsin. After Laura and co. leave the extended family, everyone gets malaria- including the pregnant mother, and then a few years later everyone gets scarlet fever. They lose an infant during the scarlet fever, and an opportunistic infection gives the eldest child cataracts, leaving her blind. Then there's the long winter, where they nearly starve. The Ingalls had five children, four who survived to adulthood; one is blind, two are weak and sickly, and only Laura manages to reproduce. Shortly after her first child is born, her husband gets mumps and becomes sterile. (To her eternal fury, that child decided not to marry.) The frigid plains were not a happy place to raise a family. But this study found when a grandmother lived within 2 miles, there were on average .6 more children per family, and an increased survival rate of 11%. It's pretty solid evidence that grandmother presence and infant survival are at least correlated. (Oh, more Little House examples. While the original family has zero descendants, the siblings of Ma and Pa who stayed near Grandma and Grandpa have a couple hundred descendants today.)

So everyone was happy and excited, and decided to do more experiments. And blast, it doesn't seem to work anywhere else. The Hazda post menopausal women work very hard, and share freely with their offspring- but there's no increase in offspring survival. And there was a recent large scale study where living grandmothers are correlated with a decrease granddaughter infant mortality, but increase infant mortality in grandsons. There was some kerfluffel about murderous paternal grannies- clearing the way for granddaughters because they share more DNA. There's not a lot of evidence for it, but there isn't really another theory for post menopausal survival either.

Let's talk about the decreased grandson fitness study! Decreasing the fitness of someone who shares 23% of your DNA seems dumb, but I'm an optimist. I'm pretty sure LeeLee (my grandmother) preferred the brother to me. She quite liked the idea of a clever granddaughter who wanted to become a botanist, but she had real affection for the quiet child who was a dab hand at bridge. (Tiny, tiny sample size. Please yell at me for bad science.)

Here's my sciencey rationalization that explains the results: say you have a gene for resistance to viruses or mental alertness or extracting rare nutrients. There's a really common version that works fantastically 99% of the time. There's a rarer version that works 90% of the time. If you have two copies of the common one, you'll be just fine 99% of the time you need to use that gene. If you have two copies of the rare one, you're stuck with 90%. If you have both- called heterosis- then 99.9% of the time, you will win. (By win I mean not die.)

What does this have to do with the previous study? Well, if this gene is on the X chromosome, grandma can be heterozygous at our theoretical allele and probably live longer than average. If a granddaughter is also heterozygous there should be a similar boost in survival. However, some grandsons could have the less awesome version of the gene, and are sadly missing a second copy of 1/26th of their DNA. It sucks to be a dude.

I think my next post will probably be on that.