Firstborns do better at school than their younger siblings but are less healthy when they are born. That is the central finding of research by Ramona Molitor and Anne Ardila Brenøe, to be presented at the Royal Economic Society''s annual conference in Brighton in March 2016. Their study analyses data on over a million children born in Denmark between 1981 and 2010.
Firstborn children are already known to do better than their siblings, but the reason for this has never been entirely clear. Some researchers believe that they receive more attention, while others believe that they are healthier at birth (which is known to be strongly linked with doing well later on).
But this study finds that firstborn children are usually less healthy at birth than second-, third- or even fourth-born siblings. Nevertheless, firstborns generally do better in their education by completing more years, doing better on tests and having higher IQs. This also happens despite the fact that women are more likely to be working, smoking and needing medical care during their first pregnancies.
? Firstborn children are less healthy at birth than second-, third- or fourth-born children from the same family.
? During their first pregnancy, women have higher labour market attachment, take more risks in terms of smoking, receive more prenatal care and are diagnosed with more medical pregnancy complications compared with subsequent pregnancies.
? The health disadvantage of firstborns is not explained by the observed differences in maternal behaviour across pregnancies.
? The firstborns'' health disadvantage stands in stark contrast to their educational achievement later in life. Firstborns perform better in school than their younger siblings – and the achievement gap increases when accounting for differences in health at birth.
Studies have shown that firstborn children complete more years of schooling, perform better on cognitive test scores and have a higher IQ than later-born children from the same family.
What explains the firstborn''s advantage in education? Some researchers have found that parents treat firstborns differently than later-born children. For example, parents monitor the homework of firstborns more and at a given age, parents spend more quality time with firstborns than with later-born children.
But some researchers have hypothesised that firstborn children do better at birth where parental influence is less important, that is, they suggest that firstborns are healthier at birth. Whether there are differences in health at birth is an empirical question.
This study empirically tests whether there are differences in the health of newborns that depend on the birth order of the child within a family and the possible mechanisms for this relationship. The researchers use rich administrative data for more than 1,000,000 children born in Denmark between 1981 and 2010.
In contrast with earlier presumptions, firstborns are less healthy at birth: firstborns weigh less, are shorter, have a lower head circumference, are diagnosed with more complications at birth and are more likely to be born premature.
Figure 1 illustrates these results, plotting birth order by family size and health of newborns, where a higher value in the health index reflects better health. While there are level differences in health at birth by family size, there is an evident positive relationship between birth order and health at birth in all family sizes.
This relationship also holds when taking account of family-specific characteristics and is not dependent on the educational status of the mother, age of the mother during the first pregnancy and gender of the child.
Women behave differently in their first pregnancy: during their first pregnancy, women have higher labour market attachment, take more risks in terms of smoking, receive more prenatal care and are diagnosed with more medical pregnancy complications compared with subsequent pregnancies.
But the disadvantage of firstborns is not explained by these observed behavioural differences as illustrated in Figure 2. The light grey bars show health of newborns by birth order. The dark grey bars do the same, but account for different maternal behaviour across pregnancies by assigning each child the mother''s behaviour of the first pregnancy.
By comparing the light and the dark grey bars, it becomes obvious that maternal behavioural differences cannot be the only reason for the disadvantage of firstborns at birth. Based on evidence from medical research and animal studies, the researchers suggest that biology might drive the disadvantage of firstborns over their later-born counterparts.
The firstborn disadvantage at birth stands in stark contrast to educational achievements later in life: firstborns perform better in ninth grade (ages 12-13) than later-born children from the same family. Once the differences in health at birth are taken into account, the firstborn advantage increases further.
These results suggest that it might be important to account for birth order differences in health at birth when analysing differences in educational achievements by birth order. Understanding how birth order differences in health evolve over childhood is a necessary next step.
Firstborns perform better in school but are less healthy at birth than their younger siblings – Ramona Molitor (University of Passau) and Anne Ardila Brenøe (University of Copenhagen)