Saturday, October 29, 2011

Bringing reproductive maturity into line with the age of marriage


Île aux Coudres, a French Canadian community on an island in the St. Lawrence

Human biodiversity is slowly making headway in academia. It has three defining principles:

1. Evolution did not end, or even slow down, with the advent of Homo sapiens. It has actually accelerated.

2. It especially accelerated about 10,000 years ago, when the rate of genetic change rose over a hundred-fold among early modern humans. This acceleration didn’t happen because they were spreading into new physical environments with different climates, topographies, vegetation, and wildlife. By then, humans had already spread throughout the world from the equator to the arctic. They were now spreading into new cultural environments with different technologies, social structures, belief systems, and means of subsistence.

3. The human species has therefore experienced more genetic change over the past 10,000 years than over the previous million years. This change has particularly involved genes for mental, behavioral, and life-history traits (Frost, 2011; Hawks et al., 2007).

One life-history trait is the age of first reproduction (AFR). Because AFR is highly heritable, there may have been co-evolution between biology and culture. In other words, natural selection has tended to bring full reproductive maturity into line with the age when young couples have enough resources to marry and start a family.

When Europeans first began to settle North America, they came from a land-poor environment where young people had to postpone marriage and family formation. Typically, they had to wait until their parents handed over the farm in whole or in part. This tendency toward late marriage was widespread throughout Western Europe, so much so that it has been dubbed the ‘Western European Marriage Pattern’:

[…] the late and non-universal marriage pattern was definitely prevalent across Northwestern Europe in the seventeenth century. By 1650, when village reconstitution studies become sufficiently numerous to render the generality of the pattern indubitable, the average age of women at first marriage was twenty-four or over, 7 to 20 per cent of women never married, and the incidence of childbirth out of wedlock was below 3 per cent. This marital pattern restricted fertility massively. A very considerable minority of women remained single and bore no children; those who married bore none for the first ten years of their fecund life-phase, on average. If they had their last child at the age of forty, their entire reproductive careers would span roughly fifteen years, a long time by modern standards but remarkably brief in a pre-transition context. Resulting fertility was less than half the rate that would have been achieved if all women between fifteen and fifty were married. (Seccombe, 1992, p. 184)

All of this changed when Europeans began to settle in the “New World.” Suddenly, land was no longer a constraint on marriage, and early marriage became the norm. With the downward shift in the age of marriage, was there a corresponding downward shift, via natural selection, in the age of full reproductive maturity?

Yes, according to a recent study of Île aux Coudres, a French Canadian community on an island in the St. Lawrence. Over a period of 140 years, from 1800 to 1940, this community saw its mean AFR fall by four years. This decline was driven not by a lowering of the mean age of marriage (which now remained stable) but by a shortening of the mean interval between marriage and first birth.

The decline in AFR seems to have been real, and not an artefact of incomplete marriage and birth records. In fact, the church registers provide exceptionally detailed birth and marriage data. Nor was it an artefact of an influx of people with lower AFRs. Almost everyone on Île aux Coudres is descended from thirty families who settled the island between 1720 and 1773.

Could the reason have been changes to diet and nutrition? Unlikely.

The advancement of age at maturity, as well as increases in fertility, may reflect plastic responses to improvements in nutritional conditions, such as those observed during the 19th and 20th centuries in Western societies. Better-fed women grow faster, mature earlier and in a better physiological state, and are more fecund. Importantly, alongside such plastic responses in reproductive traits, we would expect an increase in infant and juvenile survival rates with time. Despite some fluctuations, infant and juvenile survival rates on île aux Coudres were not higher at the end of the study period than at the beginning. (Milot et al., 2011)

When I first heard of this study, I thought that some kind of cultural lag might have been responsible. Old habits die hard. Perhaps many of the early settlers, with memories of the old country, were still afraid of not having enough land to support a family, even after they had decided to marry. This reticence would have then gradually disappeared as memories of the old country disappeared.

Such a change in mentality, however, would have happened much more among the earlier generations than among the later ones. Yet this is not what we see in the data. AFR changed at the same rate from one generation to the next throughout the 140-year period. Couples married after 1870 showed the same rate of change as couples married before 1870. Indeed, this steady rate of change seems to rule out most socio-cultural explanations, particularly those that involve some kind of re-adjustment to new conditions. In any case, the study period (1800 to 1940) postdates the years of immigration and settlement (1720 to 1773).

We’re certainly going to see more studies like this one, either from Île aux Coudres or from other regions of French Canada. In general, French Canadian communities are ideal for the study of human microevolution. Records of births, marriages, and deaths are remarkably complete over a span of three centuries, and the inhabitants tended to stay put in the same locality generation after generation. For several regions of Quebec, we already have complete genealogical databases that could be enriched with genetic data for the most recent generations.

Reference

Frost, P. (2011). Human nature or human natures? Futures, 43, 740-748.
http://dx.doi.org/10.1016/j.futures.2011.05.017

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, & R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution, Proceedings of the National Academy of Sciences (USA), 104, 20753-20758.

Milot, E., F.M. Mayer, D.H. Nussey, M. Boisvert, F. Pelletier, and D. Réale. (2011). Evidence for evolution in response to natural selection in a contemporary human population, Proceedings of the National Academy of Sciences (USA), early view

Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe, London: Verso.

14 comments:

Anonymous said...

"full reproductive maturity"
I've always assumed that a girl is biologically capable of giving birth long before she is in any sense mature. In her earliest teens a girl is capable of reproducing.

Presumably French Canadian girls in their mid teens were not rushing into marriage leaving a gap between biological reproductive maturity and the inclination to settle down. So could this falling age of first birth be due to selection for psychological traits in women, traits making them keener on marriage at a young age.

BeyondAnon said...

With a claimed closed community, there can be no influx of genes to help this along.

I suspect that there was genetic variation for age-at-first-reproduction within the original settling group.

Over time, those genes that encoded for an earlier AFR became more prevalent, as they now had a reproductive advantage :-)

You have been regaling us with much more meaty and satisfying postings over the last six months, it seems.

Anonymous said...

It's unclear to me if it is being suggested that the selection was for a genetic tendency to earlier physical maturity or psychological traits that led to earlier marriage. It would stand to reason that early maturing girls would marry earlier for a combination of reasons. But I still think that girls are capable of having children many years before they marry.

Sister Y said...

"This decline was driven not by a lowering of the mean age of marriage (which now remained stable) but by a shortening of the mean interval between marriage and first birth."

As I understand this, age of marriage plummets then remains stable; but these early-marrying girls don't reproduce until much later (though this gap declines steadily over time).

So we're not looking for early-marriage traits, we're looking for early-baby-having-while-married traits, yes?

Peter Frost said...

Anon, Anon, and Sister,

AFR did not fall because age of marriage was still falling. Age at first marriage had already fallen among the French Canadian inhabitants before the study period (1800-1940).

AFR fell because the interval between marriage and first birth was shrinking. Married women were getting pregnant faster. Keep in mind that a lot of these women were getting married as early as 15 years of age.

Beyond Anon,

Sometimes the most crucial discoveries seem rather banal ... initially.

This is all part of a new paradigm. Human evolution is not just something that happened way back in the Pleistocene. It's ongoing. English people in 1900, for instance, were biologically different from English people in 1500. We have a lot of rethinking to do, even among people who are wise to HBD.

Anonymous said...

"AFR fell because the interval between marriage and first birth was shrinking. Married women were getting pregnant faster. Keep in mind that a lot of these women were getting married as early as 15 years of age.."

I was getting a bit mixed up there, thanks for the clarification.

Anonymous said...

Frost, your responses don't really answer the same problem I'm seeing with this study. "Age at first reproduction" is an extremely broad variable. What exactly is being selected here? Fertility? Gestation periods? Physical/mental maturity? Because this study seems to be making a huge fuss over a decline of 4 years over a 140 year period. For an anecdotal case, my mother was in her mid 30's when I was born, whereas my grandmothers were both in their early 20's when they gave first gave birth.

In fact, the trend towards later births is has been widely noted across the world, so what exactly is this study saying?

Peter Frost said...

Anon,

"What exactly is being selected here?"

- Increased fecundability at earlier ages.

"the trend towards later births has been widely noted across the world, so what exactly is this study saying?"

The trend towards later births today is due to later ages of marriage and the use of contraception within marriage.

Neither factor was operational here. Married women were getting pregnant faster. There was a shortening of the time between marriage and first birth.

Anonymous said...

But how were they able to pin this to an down an increase in biological fertility? Again, this was a small decrease in age over 140 years.

Anonymous said...

Peter said:

Sometimes the most crucial discoveries seem rather banal ... initially.


Well, yes. However, how do you distinguish between a facultative behavior (that humans have evolved to use environmental queues when making decisions about when to have children) vs strongly genetically controlled behavior with different frequencies for different behaviors and their selection by the environment? I had the same questions about the claims some time ago that girls were facultatively choosing to be sluts or paragons of virtue based on the presence or absence of a father early in life. It seems rather more likely that the explanation for girls' behaviors is that they are inheriting the genes of their mothers (and fathers).


This is all part of a new paradigm. Human evolution is not just something that happened way back in the Pleistocene. It's ongoing. English people in 1900, for instance, were biologically different from English people in 1500. We have a lot of rethinking to do, even among people who are wise to HBD.


In a perfectly benign environment, intelligence and delayed gratification is not much use, and even if they did evolve, they would be outbred by those with just enough intelligence and future time orientation to get by with.

Anonymous said...

Scientific American also reports on something similar

Peter Frost said...

Anon,

Do you have an alternate explanation? I was initially skeptical myself.

A 4-year decline in age of first birth can make a big difference in the total number of children a woman will have. Women do not have a very wide time window for childbearing. From the age of 30 onward there is a measurable decine in fertilty and fetal viability.

Anon,

"In a perfectly benign environment, intelligence and delayed gratification is not much use, and even if they did evolve, they would be outbred by those with just enough intelligence and future time orientation to get by with."

Well, yeah. But until the 20th century very few people lived in a perfectly benign environment. You might find this hard to believe but the middle class used to outbreed the poor.

Beyond Anon said...

Peter said:

Well, yeah. But until the 20th century very few people lived in a perfectly benign environment. You might find this hard to believe but the middle class used to outbreed the poor.


Oh, I believe it ...

BTW, many of the animal species we consume live in a perfectly benign environment until we harvest them. Some lineages have pretty good reproductive success as well.

Anonymous said...

Perhaps this is another indication of this phenomenon:

"10-year-old Mexican girl gives birth to baby boy after 31-week pregnancy"

http://www.nydailynews.com/news/world/10-year-old-mexican-girl-birth-baby-boy-31-week-pregnancy-article-1.975740

A 10-year-old Mexican girl has given birth to a baby boy after a 31-week pregnancy, according to reports.

The premature infant, which weighed 3.3 pounds, was born by Caesarian section at the Women’s Hospital in the city of Puebla and is in the intensive care unit recovering from pneumonia.


...

This is not the first case of a shockingly young girl giving birth in Mexico.

In August last year, 11-year-old Amalia had a child two weeks premature after she was denied an abortion by the local Justice Department during the fourth month of pregnancy.


This is just anecdotal, of course, but there have been studies suggesting that girls are entering puberty earlier:

"Study: More U.S. girls starting puberty early"

http://www.cnn.com/2010/HEALTH/08/09/girls.starting.puberty.early/index.html

Girls in the United States are entering puberty at earlier ages than they have in the past, a new study reports.

More than 10 percent of white 7-year-old girls in the study, which was conducted in the mid-2000s, had reached a stage of breast development marking the start of puberty, compared to just 5 percent in a similar study conducted in the early 1990s.

Black and Hispanic girls continue to mature faster than white girls, on average. Nearly one-quarter of black girls and 15 percent of Hispanic girls had entered puberty by age 7, according to the new study, which appears in the journal Pediatrics.


This would seem to be dysgenics in action. Can a decline in AFR generally be considered dysgenic?