Saturday, December 19, 2020

Brain size and family structure in Estonia


Estonian schoolchildren (Wikicommons). Estonian children have smaller brains if raised by a biological parent and a step-parent. Therefore, two committed parents are better than one, right? Well, not in this case. Brains aren't smaller in Estonian children raised by a single parent (and no step-parent).



In Estonia, cranial volume was one of several anthropometric traits that were routinely measured in schoolchildren during the Soviet era. The data didn't suffer from volunteer bias because the measurements were mandatory. Mortality bias was minimal because the subjects were young. This data source is thus better in many respects than data from Western biobanks. It is now being mined by Peeter Hõrak, a University of Tartu professor, to learn more about nature and nurture in human brain development.


I discussed this data source in a previous post (Frost 2020). One problem is that the study population is not as homogeneous as it may seem. In fact, 16% of the fathers and 7% of the fathers were not Estonian (Hõrak 2020). This factor might explain some differences in the data, especially changes over time.



The latest study


This data source has now been used to see whether the brain size of children is influenced by family structure, specifically whether the child was raised by biological parents or by step-parents. The data came from 822 children born between 1980 and 1987 in Tartu, Estonia and were measured at around 14 years of age.


The children had significantly larger brains when the household had both biological parents:

Cranial volume was related to family structure and paternal education. Children living with both birth-parents had larger heads than those living in families containing a step-parent. [...] our findings suggest that families including both genetic parents provide non-material benefits that stimulate predominantly cranial growth. (Lauringson et al. 2020)


That's what we read in the Abstract. The brain was bigger on average in children who had been raised by both biological parents, rather than by a biological parent and a step-parent, presumably because a step-parent contributes less to the child's upbringing.


That finding is rejected, however, in the Results section. It turns out that there was no difference in brain size between children raised by both biological parents and children raised by a single parent (in almost all cases the biological mother). The brain was smaller only in children raised by a biological parent and a step-parent:


At the same time, cranial volumes of children living with a single parent were similar to those living with two providers, even though the former reported on average lower resource availability and more frequent meat shortage. Associations between family type and cranial volume thus cannot be explained on the basis of dilution of material resources. (Lauringson et al. 2020)

Differences in family structure also failed to correlate with differences in the child's height. If life in a stepfamily had somehow harmed the child's development, that harm was much less observable in overall body growth than in cranial volume.


So what's going on here? Keep in mind two things about Estonian society of the late 20th century:


- A single parent was almost always a woman, often a widow who refused to remarry, either because she still felt attached to her deceased spouse or because she considered the potential husbands available to be more trouble than they were worth.


- A step-parent could be of either sex. A stepfather often took over from a man who had sired the child out of wedlock or during a short-lived marriage.


Thus, on average, the biological father was a different kind of man in the two situations. In the first situation, he was usually the sort of man who would remain with the mother of his child until his death. In the second, he was often the sort of man who would leave the mother of his child once a more interesting woman came into view. One may presume there are differences in genetic quality between the two kinds of men. This hypothesis is actually advanced in the study:


An alternative (yet not mutually exclusive) explanation to the observed associations between family type and cranial volume of children would be that parents prone to remarrying possess on average (genetically) smaller heads than those prone to avoiding divorce or remaining single after divorcing. Such a scenario would assume robust genetic correlations between cranial volume and personality traits related to marriage stability. Twin studies have shown that genetic factors account for 13-53% of the variation in divorce [...], and if personality traits associated with a propensity to divorce are genetically correlated with cranial volume or its growth rate, one would detect smaller heads of children growing up in divorced/separated families. Such an explanation would be consistent with the predictions of life history theory, assuming that qualities characteristic of slow pace of life-including high somatic investment into body and brain growth and propensity for relatively low mating effort (in relation to parenting effort)-have coevolved (and cluster) with higher mental abilities and conscientious and risk-averse personality traits [...]. Consistent with this view are also the findings in our sample where fathers with only primary education were shorter and more prone to divorce/separate than others. (Lauringson et al. 2020)


We see a similar problem of interpretation with the relationship between father absence and early sexual maturity in daughters. Using a large sample of 1,247 daughters, Surbey (1990) found that daughters with an absent father matured four to five months earlier than those who lived with both parents continuously and seven months earlier than those with an absent mother. Surbey argued that the presence of a strange male accelerates the speed of sexual maturation. In other words, at a subconscious level, the girl does not recognize the man as a father. She recognizes him as a potential mate, and her body gears up for procreation.


This hypothesis was challenged by Mendle et al. (2006) who examined the daughters of twin mothers.


In a pair of twin mothers of which only one raises her children with a stepfather, the offspring of both twins are equally likely to display early age of menarche. It therefore appears that some genetic or shared environmental confound accounts for the earlier association found in female children living with stepfathers.


It seems, then, that people who end up as step-parents are, on average, genetically different from other parents. They tend to have the mental and behavioral characteristics of a "fast" life history.





Frost, P. (2020). Declining intelligence in the 20th century: the case of Estonia. Evo and Proud, August 3


Hõrak, P. (2020). Personal communication.


Lauringson, V., G. Veldre, and P. Hõrak. (2020). Adolescent Cranial Volume as a Sensitive Marker of Parental Investment: The Role of Non-material Resources? Frontiers in Psychology 15 December


Surbey, M.K. (1990). Family composition, stress, and the timing of human menarche. In T.E. Ziegler & F.B. Bercovitch (eds.) Socioendocrinology of Primate Reproduction, pp. 11-32, New York: Wiley-Liss Inc.


Anonymous said...

"Surbey (1990) found that daughters with an absent father matured four to five months earlier than those who lived with both parents continuously and seven months earlier than those with an absent mother. "

What effects does earlier maturity have on the aging of an individual later in their life?

Peter Frost said...

A faster life history is associated with earlier aging.

Anonymous said...

A Scandinavian colleague showed me a video of a Swedish high school graduation, I was struck how they look like older adults. The colleague, still a young woman, looked nearly middle aged herself.

But I've also been seeing more people who look very young for their age, in childhood and later life. However, better medicine/nutrition could mask a shift to a faster life history, whose effects will one day take us by surprise. said...

Probably, there is a difference is preferences between a woman (1) that marry after having a baby out of wedlock or a short marriage and one (2) that doesn't remarry.

I could suppose the woman in case (1) have lower time preferences and it is not very picky about the mate she choose where (2) is pickier and have higher time preferences.

The same probably works in the reverse:
A good husband material would be pickier on the woman he choose than other men.

So they sort themselves by time preferences.
And time preference is liked to intelligence and intelligence to brain size. said...

It would be interesting to check the brain size differences between male and female children in step-parent / single parents families compared with the both biological parents family.

Could the presence of a same sex (as the child) step-parent influence the development of the child?
The presence of a step-mother inhibit the maturation of the daughter, a step-father could inhibit the maturation of the son. The inhibition could be useful to prevent competition with the dominant male / female

Anonymous said...

once again, these (abstract) studies suffer from referring to concretre situations of living/housing under the harsh conditions of 'real socialism' - especially in the former USSR. in most cases, a flat was only available for married couples with a child - otherwise the daughter/son had to stay with the parents (often plus a grandparent/aunt in the minuscule flat!)
little wonder, that girls tended to grasp the first man available for marriage and getting pregnant from him, nevermind he was a drunkard - just to get the permission for a new flat in the building... these marriages seldom lasted for long - hence so many cases of stepfathers/patchwork families there.
small brainsize certainly very often caused by alcohol abuse by the pregnant mother: life in USSR was drenched in alcohol for both sexes.

Michel Rouzic said...

The elephant in the room here seems to be that the brain sizes of the biological parents were either not measured or not taken into account, so we miss a key piece of the puzzle to determine anything with any degree of certainty. To hypothesise that the environment leads to a smaller brain first we'd have to see if there's a discrepancy in the brain size of the child from what you'd expect given the brain sizes of the biological parents, not just if the brain sizes are different between children living in different situations. It's a rather major methodological flaw. It seems like similar flaws are more often than not present in studies that attempt to link environmental factors to cognitive performance, like the idiotic studies that conclude that American inner city poverty magically makes the brains of infants shrink even only a few months after birth. Nature itself presented such a study using the abysmally stupid headline "Poverty shrinks brains from birth", and the article includes a suggestion that the lack of stimulating toys or stress-related "epigenetics" might be to blame... They'd rather resort to superstitious explanations than accept that genes might account for anything.