A new study argues that in-law competition drove the evolution of menopause. But is the story too good to be true?
In the classic Scandinavian folktale "The Twelve Wild Ducks," also known as "The Twelve Brothers" in Grimms' Fairy Tales, a wicked old Queen is jealous of her daughter-in-law's beauty. In a rage of envy she kidnaps her own grandchildren during the night and throws them into a snake pit. She then tells her son that his young wife is actually a witch who has eaten their children and demands that she be burned at the stake. In the end, the daughter-in-law gains the upper hand and the wicked Queen is bound between twelve wild horses who rip her body into pieces.
Such stories were widespread in Scandinavia and northern Germany during the 18th and 19th centuries. A common theme in many of them--especially the more popular ones such as "Snow White," "Cinderella," or "Sleeping Beauty"--is of a beautiful damsel who is persecuted by an older mother-in-law or stepmother who mourns her own lost youth. Invariably her plot fails and the evil matriarch is sent to a grisly death. Similar tales can be found in Russia, India, and Japan. For an 18th century peasant the moral could not have been more clear: intergenerational conflict leads to disaster. But could there be something deeper behind the "collective unconscious" found in so many of these tall tales?
"Evolutionary biological insights yield a powerful set of instruments with which to understand literature and, in the process, ourselves," write David and Nanelle Barash in their book Madame Bovary's Ovaries: A Darwinian Look at Literature. According to Barash and Barash, these intergenerational conflicts between women may be so common in our folk traditions because they are part of a common biological heritage.
Now, in a new study published last week in the journal Ecology Letters, biologist Mirkka Lahdenpera of the University of Turku in Finland and colleagues argue that this conflict between female in-laws has a very real impact on child survival. What is more, they conclude that this intergenerational reproductive competition is a leading factor behind the evolution of menopause.
By utilizing Finnish church records from between 1702 and 1908 the researchers were able to assemble a remarkable data set that detailed every birth, death, marriage, and migration in five separate communities across three generations. Their analysis revealed that when women and their mothers-in-law had children within two years of one another infant survivorship was reduced by 50% for older women and 66% for daughters-in-law. In contrast, there was no decline when women and their own mothers reproduced within the same time frame.
"Our results confirm that intergenerational conflict between in-laws can be intense," the authors conclude, "with substantial reductions in the survival of offspring from mothers of both generations." They explain this by citing William Hamilton's theory of inclusive fitness, that individuals will tend to cooperate more with those whom they share more genes.
As the result of meiotic cell division in sexual reproduction a woman's mother-in-law would be expected to share 25% of their genes with her grandchild (assuming that her son is actually the father) but would share 50% with her own children. In an environment where there are limited resources and two infant mouths to feed in the same household the struggle for existence between in-laws could become fierce. Mothers may have then found themselves playing out the same roles as those in the folktales they grew up listening to around the fireplace.
However, while this first interpretation has strong empirical support, where the story goes next becomes somewhat more fanciful. Because of the significant decline in infant survivorship that occured when in-laws had children at the same time, Lahdenpera and colleagues argue that it would therefore be adaptive for women to stop ovulating before their daughters-in-law began having children themselves. The limited reproductive overlap seen between generations in our recent past is but an echo of the stabilizing selection pressure that occurred long ago over evolutionary timescales. Menopause at around 50 years old was the ultimate reproductive compromise.
In this way Lahdenpera's Reproductive Conflict Hypothesis of menopause is the sinister mirror image of what is currently the leading interpretation for why women have such lengthy post-reproductive lives. The Grandmother Hypothesis, developed by University of Utah anthropologist Kristen Hawkes, argues that long post-reproductive lifespans were selected for during our evolution because it allowed grandmothers to help their own children to successfully rear the next generation. Previous studies have made a strong case for the Grandmother Hypothesis, including one by Lahdenpera that appeared in Nature in 2004.
This earlier study found that the presence of either a mother or a mother-in-law resulted in significantly more grandchildren surviving to adulthood. It was, Lahdenpera wrote, "the equivalent of post-reproductive women gaining two extra grandchildren for every ten years that they survive beyond age 50." However, eight years later, it is unclear how the same data could generate seemingly contradictory findings. For evolutionary researchers like Hawkes it mars what is an otherwise "strong and provocative result."
"I admire the work of this research group and the ways they've used this Finnish data set to explore questions about human life history," Hawkes told me. "That said, in their 2004 paper they found positive grandmother effects through both sons and daughters. What's going on here? They say these Finns were very monogamous and that both sons and daughters either would not or could not disperse."
This issue of dispersal and residence patterns could be a serious problem for the Reproductive Conflict Hypothesis. A sedentary farming village in 18th century Finland is far removed from the migratory lifestyle of our hunter-gatherer ancestors. While Lahdenpera and colleagues acknowledge this in the introduction, they do not seem to take the implications fully into account.
Modern hunter-gatherers, and presumably our distant ancestors as well, are often what anthropologists refer to as multilocal in which individuals migrate between their husband's or wife's kin depending on where they can receive the most help. It has even been shown, for example among the Hadza in Tanzania, that grandmothers will travel to where they can be most useful. This dynamic would seem to undermine support for intergenerational reproductive conflict. If conditions were too onerous with the mother-in-law young families could simply "vote with their feet." At the very least it complicates the assumptions that Lahdenpera and colleagues included in their model.
"This is exactly why assumptions we make about behaviors that are or are not adaptive can be so problematic," University of Illinois anthropologist and Scientific American blogger Kate Clancy told me. Settled agricultural populations might simply be subject to more conflict between the generations whereas foragers would have much less. Furthermore, unlike these Finnish communities, only 14.5% of modern preindustrial societies can be classified as monogamous (though they would still include some monogamous individuals). The idea that a young woman could be beholden to her wicked mother-in-law would certainly not be as common among our hunter-gatherer forebears as it was for the contemporaries of Jacob and Wilhelm Grimm.
"Rather than this study demonstrating adaptive value for post-reproductive life," Clancy said, "it seems to provide more evidence that moms and their mothers-in-law had legitimately different reproductive strategies that can sometimes be in conflict." If the grim tales of murder and deceit between generations of women do have a basis in biology it will probably be this more limited interpretation. For now, at least, it would appear to be the Grandmother Hypothesis that lives happily ever after.