Having a big family makes your children either badly behaved or low achievers at school, study claims

The academic article underlying the popular report below is "The Quantity-Quality Trade-off and the Formation of Cognitive and Non-cognitive Skills". In my usual pesky way, I have had a look at it

I don't have the time to look in great detail at this quite  complex study so I will content myself with a couple of basic observations.  For a start, the finding is unusual.  Other studies have found no effect of family size.

The problem, if there is one, appears to be an artifact of social class, though the authors are not allowed to mention that naughty word, of course.  The effect seems mostly found among the poor, who are also less bright and who are also more likely to have big families.

And here's the tricky bit: Mention of poverty in America immediately calls to mind the naughtiest word of all: race.  Did  the authors control for race?  Would the effect drop to insignificance if you looked at whites only?  The Abstract of their current paper does not mention that word. But here's the kicker.  There is also online what appears to be a preprint of the paper.  And that DOES mention the word.  And they DID find that race had a big effect.  The effect of family size was primarily seen among persons of sub-Saharan African ancestry ("blacks" in non-academic language).

And the effects overall were not large.  The word "IQ" is another word that may not be mentioned in polite circles.  It is too easily understood.  But their statistics can be translated into IQ.  And the result is that we are looking at only about an IQ change of 1.5 IQ points.  So the whole thing hardly matters anyway.

The takeaway is that in most families parents can have as many children as they like without concern about dumbing their kids down

Finally: I don't like to do this but I feel that I must place this study in the context of the current uproar in psychology about the high rate of unreplicable results and the associated topic of research ethics. It is now clear that many scientists do not tell the full truth about their research results -- for various reasons. 

In that context, any concealment of findings calls into question the integrity of the research and the integrity of its authors.  And since scientific communication depends heavily on trust, any attempt at concealment of findings -- as we see in the published abstract of this study -- strongly suggests that the work was not honestly reported and should therefore be disregarded.  I am not being cynical in saying that the abstract IS the article for most readers of  academic journal articles.  Only specialists in that field plough through the whole thing

In the circumstances it is open for one to conclude that the real findings concerned blacks only but that was too unpalatable ("politically incorrect") to publish.  So that problem was "worked around" in one or more ways


A new study has found that for every additional child born, the others are more likely to suffer poor cognitive abilities and behavioural problems afterwards.

Boys were more likely to misbehave while girls saw their performance in maths and reading skills dip.

Using data from 1986 to 2012 taken by the National Longitudinal Survey of Youth (NLSY) and by the Children and Young Adult Survey, three economists analysed how older siblings performed before and after a younger sibling was born.

They looked at the number and timing of births into a family and matched these to various mental and behavioural traits.

Levels of parental engagement were also crucial - with factors like how often families eat meals together, one-on-one time with each child, affection and the safety of the home also affecting how a child performed.

As families got bigger, the time spent with each child reduced, which has been linked to worse outcomes for children, they found.

'Our fixed effect estimates indicate that the arrival of a younger sibling reduces measures of parental investment as well as cognitive and non-cognitive outcomes of older children by approximately one-tenth of a standard deviation,' the research paper said.

The study was conducted by economists Chinhui Juhn, Yona Rubinstein, and C. Andrew Zuppann, who questioned whether the 'quantity' of children would effect the 'quality' of their upbringing. 

It discovered that parental investment in older kids fell by 3 percentile points after a young child is born, while cognitive scores fell by 2.8 percentile points and behavioural problems increased.

'We have documented a significant trade-off between quantity and quality of children for NLSY mothers and their children. 'On average, children in larger families have lowered parental investment and worse cognitive and non-cognitive outcomes. '

Other factors found to influence the outcomes was the mother's intelligence and economic well-being.

Mothers were asked to take the Armed Force Qualification Test (AFQT), used by the military to assess skills including reading and reasoning.

Those who scored badly saw a larger drop in cognitive scores when they had their second child.

SOURCE