Women's Traveling Ways Written in Thai Tribes' Genes

Hillary Mayell
for National Geographic News
May 10, 2005
A new report shows how genes can help reveal how societal rules affect mobility. The genetic study focused on traditional tribes in Thailand and found that in some societies, many more women than men migrated for marriage. One explanation is that men have been keeping non-related men out of their villages.

The findings raise questions about how society structure can impact genetic diversity in certain locations. This kind of research may also lead to the use of genetic analysis to determine early society structure.

The researchers studied six traditional populations in northern Thailand to determine the effect social organization has on the migration patterns of men and women.

Three of the groups were patrilocal societies, societies where married couples move to the husband's village. Three more groups were matrilocal, societies where married couples typically live in the wife's village.

The researchers found that the two societal arrangements result in significantly different postmarital migration patterns, despite the fact that the groups are all agriculturally based and live in the same geographic region.

"What we find is that in the patrilocal societies, the females are moving much, much more than the males are," said Laurent Excoffier, a population geneticist at the University of Bern in Switzerland.

"In matrilocal society males are moving a bit more than females, but it is not at a rate that is statistically significant—essentially the males and females are moving at about the same rate. It's clearly different from what is happening in the patrilocal society."

The hill tribes studied included the Ahka and two groups of Lisu, representing a patrilocal society. The Lahu, Red Karen, and White Karen represented matrilocal societies.

All six groups are agricultural, hacking a difficult livelihood out of marginal lands in the isolated backcountry of Thailand.

The study showed that, in patrilocal societies, 15 times more women than men were being exchanged among these neighboring populations.

"Our findings raise interesting questions about what is controlling male-versus-female migration in these groups," said Mark Stoneking, an evolutionary geneticist at the Max Planck Institute for Evolutionary Anthropology in Germany.

"The fact that the matrilocal groups are fairly loose and that, even though they prefer that males migrate to the residence of the female, there's obviously a lot of female migration still going on. ... In patrilocal groups they've basically completely shut down the migration of males between groups. [That] is compatible with the hypothesis that men are strictly controlling male immigration in patrilocal societies."

Tracking Genetic Data

Using previously published genetic data to track migration patterns, the researchers developed a statistical model to examine the differences in mitochondrial DNA (mtDNA) and Y chromosome data among the six groups.

Population geneticists study mtDNA and Y chromosome data to determine an individual's genetic history.

Mitochondrial DNA is passed on from mothers to both sons and daughters. Y-chromosomes are passed only from fathers to sons.

Every so often random mutations in a DNA sequence occur. These mutations, which happen naturally and are usually harmless, are called markers.

Once a marker has been identified, geneticists can count the differences within and among populations, defining a unique lineage of descent. These lineages can be used to track migration patterns.

Prior to this study it was not possible to estimate male and female migration separately. The genetic data allowed the authors to compare the movements of males and females within the same society.

What they found is that among the hill tribes of northern Thailand, on average, fewer than one male entered a patrilocal society per generation, but there were lots of new females—more than seven per generation.

The ratio of female immigrants versus male immigrants was much more balanced in matrilocal hill tribes. Here, the number of incoming men and women was close to the same, with around four males and three females entering per generation.

The study raises interesting questions about how social organization affects genetic diversity and suggests that genetic data can be used to interpret social organization in the past. The study is published in this week's issue of the Proceedings of the National Academy of Sciences.

Looking at Prehistoric Societies

Future studies may enable scientists to look at genetic data and determine the social organization of a particular group.

"One idea is that, perhaps earlier in human history, most groups were matrilocal or at least there wasn't a great deal of control over whether males or females migrated," Stoneking said.

According to the theory, only when resources became abundant did patrilocality arise. Stoneking said that by staying put and keeping other males out, men controlled those resources.

Under this hypothesis, patrilocality would emerge, depending on the region, between 6,000 and 12,000 years ago, as groups converted from hunting and gathering societies to agricultural societies.

"What we'd known before is that in human populations, we find, in general, bigger differences between groups for the Y chromosome than for mitochondrial DNA, which is consistent with patrilocality," Stoneking said. "In this study we were able to test what we already knew and to actually estimate male and female migration rates."

"The novel conclusion of this study is that patrilocality and matrilocality are not just simply the opposites of each other," he said. "There's a lot more going on in terms of social control of movement of individuals. ..."

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