Sisters and brothers: it’s complicated

A Finnish family of the type discussed in this study. You can’t see it, but there’s sibling rivalry going on. Source: Dr Virpi Lummaa family album.
Family life just got even more interesting. Just in time for Christmas, too!

Families bustle with the push and pull of conflict and cooperation. But just how profoundly in the end do family members effect one another in the only currency that really counts: evolutionary fitness? A new study out of Finland via Sheffield suggests the effects can be both profound and complex.

I’ve written here before about the work of Virpi Lummaa and her colleagues analysing Lutheran parish records in pre-industrial Finland. These detailed and comprehensive records, painstakingly entered into computer databases, now present an exceptionally rich source of quantitative information about life and the effects family members have on one another. Their results have, among other things, revealed much about the evolutionary importance of grandmothers and the conflicts between women at the threshold of menopause and their daughters-in-law.

I’m usually reluctant to return to the work of a single group too often, but the latest results from this project are just too cool to pass over.

This week in Proceedings of the Royal Society of London B, they turn their attention to the intriguing effects that siblings have on one another’s evolutionary success. Aïda Nitsch, Charlotte Faurie and Lummaa set out to find if siblings are helpers and competitors. The answer: both of the above.


They used records of the all 9585 daughters and 10106 sons born to 3829 mothers between 1750 and 1900. That’s an average of over five children per family, providing plenty of data about the costs and benefits of having siblings. The paper focuses on the effects of older siblings.

One in three children died before reaching fifteen years of age, usually from infectious diseases – often associated with malnourishment. But the more older siblings a child had, the greater the chance that child would survive to 15. In the most dramatic instance, having four older sisters raised a boy’s chances of living to adulthood by about ten percent.

Interestingly, the weakest effect was that of elder sisters on girls. The authors suggest that this might be because girls contributed in and around the home whereas boys’ main contribution was on the farm. So all children shared the benefits of older brothers’ work, whereas older girls might have channelled more of their efforts toward younger brothers than toward sisters.

And this exposes the sibling rivalry. Sixty five percent of boys and 75 percent of girls who made it to 15 ended up marrying and having children. Having opposite-sex elder siblings didn’t change the chances of marrying or the number of children an individual had. But same sex elder siblings made the mating business appreciably worse.

A boy with no older brothers had a ten percent greater chance of marrying, and ended up averaging one more child in his lifetime than a boy with four older brothers. The effects were not quite as dramatic, but nonetheless significant for girls with four elder sisters. They bore four children, on average, whereas girls with no elder sisters averaged about 4.9 children in their lifetimes.

In small farming communities, the mating markets would have been very small. A sibling who takes one eligible mate off the market by marrying them can have an appreciable effect on one’s chance of ever finding a mate. On top of that, farms passed from parents to their eldest sons. This primogeniture made the second and subsequent surviving son a far less attractive mate.

This paper illustrates the complex cooperative conflicts that evolutionary biologists and economists have been discovering at the heart of family life. And intriguingly, the conflict wins out over the cooperation. When the positive effects of siblings on childhood were combined with the reduced mating success that came from of having same sex siblings competing for mates, competition outweighed cooperation. Continue reading Sisters and brothers: it’s complicated

Is human intellect on the downward slide?

Did human intellectual capacity peak 600 years before Plato? Raphael’s Scuola di Atene fresco in the Vatican, 1511. Wikimedia commons

I would wager that if an average citizen from Athens of 1000 BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive of our colleagues and companions, with a good memory, a broad range of ideas, and a clear-sighted view of important issues.

So Stanford geneticist Gerald R. Crabtree begins back-to-back Forum pieces for Trends in Genetics, entitled “Our Fragile Intellect” (Parts I and II). Crabtree’s thesis: humanity is “almost certainly” losing its superior intellectual and emotional capacities.

Crabtree doesn’t seem to be arguing for the intellectual vibrancy of the Akademia or the Lyceum. These places, and their celebrated occupants like Plato and Aristotle graced Athens only 600 years later, well beyond Crabtree’s inferred date of humanity’s intellectual zenith.

And he doesn’t confine himself to Athens. “I would also like to make this wager”, he goes on, “for the ancient inhabitants of Africa, Asia, India, or the Americas, of perhaps 2000-6000 years ago.” He’s arguing that humans – throughout the world – have been steadily losing their marbles for the last three to six millenia.

Well, Professor Crabtree, I’ll see your Athenian intellectual Titan. And I’ll raise you a bottle of 1998 St Henri and a $100 book voucher.
Continue reading Is human intellect on the downward slide?

China’s biggest problem? Too many men.

Sydney (CNN) — In the mid-19th Century, two devastating floods of the Yellow River, and the famine that followed, ravaged northeastern China.

Outlaw bands, known as nien, attracted young men in unprecedented numbers, aggregating into militias that wrought chaos on the troops and infrastructure of the ruling Qing. Although this Nien Rebellion and the larger Taiping rebellion in the South were eventually crushed, they devastated the Chinese economy and contributed to the ending of the Qing dynasty.

According to political scientists Valerie Hudson and Andrea den Boer, widespread female infanticide during the famine meant that as many as one quarter of young men in the region were “bare branches” — as the Chinese expression goes — unlikely ever to bear fruit. The Nien rebellion, they argued, was propelled by these surplus young men who had so few other prospects.

This story of the Nien Rebellion foreshadows one of the biggest issues that China will face in coming decades: the dramatic excess of young men.


A memory of Mitch Lucker

I am unexpectedly moved this morning by the news that Mitch Lucker has died following a Halloween motorcycle accident.


Mitch Lucker (1984-2012) performing with Suicide Silence Supkowski on Flickr


Mitch was the “vocalist” for the American metal band Suicide Silence. I qualify the word “vocalist” because such is Suicide Silence’s extreme brand of metal that he more growled than sang.

Mitch was 28 years old. Very much in the “danger zone” for popular musicians, especially male musicians. I have written before about how the fast-living and risk-taking path to the top in genres like rock and rap disposes musicians to an early death.

Those who reach the pinnacle are five times more likely to die than their contemporaries of the same age and nationality. But we have no objective data on how many die trying or die in failure. I would predict that the few famous deaths that catch our attention are the tip of a much bigger iceberg.

I am no fan of Suicide Silence’s music. I am moved nonetheless by Mitch’s death because I had the immensely good fortune of sitting among members of the band and enjoying Mitch’s company on a flight from Sydney to Brisbane last year. I had just published Sex, Genes and Rock ‘n’ Roll, and I was on my way to give one of the first talks to promote the book. Continue reading A memory of Mitch Lucker