He discovered that the educational level of non-biological mothers is a significant determinant of their children’s educational level, while there is no such link between the educational level of non-biological fathers and that of their children.

‘We applied a methodology that had never been used before.’ That is the simple reason why the Review of Economic Studies was interested in his research, according to Erik Plug. Social scientists have been examining the relationship between the educational levels of parents and their children for many years now. In doing so, they often investigate whether a similar educational level between parents and children is caused more by genetic predisposition (nature) or by upbringing (nurture). The population under study in such research often consists of twins or adopted children.

Exciting results

In his research, the professor used information on IVF treatments from Denmark, where they are tracked very closely. These treatments involve parents who use the sperm or egg cells of others to achieve pregnancy. As Plug explains: ‘We had a look there to see how children conceived in this way develop in terms of educational outcomes. We wanted to know whether the education of non-biological parents is a significant determinant of their children’s achievements. The study has produced some exciting results. The method is very novel and, in my eyes, an improvement on what we had before.’

It is indeed an impressive finding: the educational level of non-biological mothers appears to be a significant variable in determining their children’s educational level. By contrast, the educational level of non-biological fathers has no effect on their children’s test scores. So children conceived via egg cell donation do better at school when their mother is well-educated. This cannot be explained by genetic transfer but must be caused by other factors. To conduct his research, Plug used information about IVF treatments in Denmark relating to 820 children conceived from the sperm of male donors and approximately 160 children conceived from the eggs of female donors.

Unequal opportunity

Why are the results of nature-nurture studies so important to economists? Plug points out: ‘It’s very helpful in figuring out why unequal opportunity is so persistent. Children born to well-educated parents often end up better than children whose parents are less educated. Policy makers are keen to know what they can do about it: How important are parents for the opportunities available to their children? Unequal opportunity appeared to be genetically driven more than anything but it seems that the mothers’ education also plays a key role. If it’s good, the educational performance of their children will also be good.’

Adoption versus IVF

Why well-educated mothers have such a strong influence is more difficult to ascertain. Do they create a better environment for their children, allowing them to learn in peace? Are better-educated mothers more passionate? Plug compared the results with data of parents whose children were adopted. In the latter case, the correlation between the education of non-biological mothers and that of their children was negligible and very similar to the corresponding relationship for non-biological fathers. ‘It therefore seems that non-biological mothers of IVF children have the strongest impact when their children are very young. Adopted children often don’t join the family until they’re between 6 months and 2 years old. It could even be the case that antenatal experience in the womb is important for the development of the child. As an example, well-educated mothers are perhaps less inclined to smoke during pregnancy so that they then have a favourable effect on the educational achievements of their children.’

Career opportunities for mothers

This is by no means the only study where Plug works with IVF data. Another line of research looks at how motherhood affects career progression. ‘We think, for instance, that children are an important reason why men earn more than women. In 9 out of 10 cases, it’s the mother who’ll take care of the children. She’ll then often work shorter hours or get another job. This could have long-term consequences: she’ll miss out on work experience or end up on a different career path. Here too, IVF data can give us greater insight.’

Does the education of non-biological mothers also affect the careers of their children at a later stage? Plug’s research has not yet addressed that question as it focuses on educational outcomes. ‘Although educational achievements can be a predictor of subsequent success, it may well be that non-biological fathers actually play a more important role when the children are older. In the early years, mothers are often the ones who take on the role of child-rearing. In later years, fathers are the ones who can support their children as they make career choices. That’s something I’d like to look at in about 10 years’ time!’

About Erik Plug

Erik Plug completed a master’s degree in econometrics from the University of Amsterdam in 1992. He then worked towards a doctorate at the Tinbergen Institute, obtaining his PhD from UvA in 1997. He currently holds a professorship there.