22(30)
7
Sibling comparisons
In order to shed light on the importance of social factors in the context
of sickness absence, we compare the sickness absence between siblings
with the same biological parents. More precisely: we compare the sickness
absence level among firstborn children and their younger siblings of the
same gender.
There is a large literature showing that firstborn children are more
successful on the labour market than their younger siblings (see, e.g.,
Devereux, Black and Salvanes, 2005; Sulloway, 2007). Furthermore, this
difference cannot be biologically determined; rather, it seems as if it is the
social rank within the family during childhood that explains the difference
(Black, Devereux and Salvanes, 2011; Kirstensen and Bjerkedal, 2007).
20
Our conclusion is, hence, that any potential difference in sickness absence
between firstborn children and their younger siblings stems from social
factors rather than a difference in genetic predisposition to health
problems.
In order to follow the preceding analysis, we relate the potential difference
in sickness absence between siblings to the level of sickness absence
among their parents. We ask whether the importance of social rank for
explaining any potential sibling differences in sickness absence varies
depending on whether the parents have any sickness absence or not.
The results from this analysis shed some light on the importance of social
factors for explaining sickness absence. An earlier result in this paper has
shown that children with parents with sickness absence in general are more
absent due to sickness themselves than children with parents without
sickness absence. If the sibling difference due to social rank varies in
importance depending on the level of sickness absence within the family,
we gain a hint about the relative importance of joint family components
versus social rank during childhood for explaining sickness absence.
All the comparisons are made between siblings of the same gender. The
reason is that sickness absence differs between genders and the birth order
may depend on the gender composition among the siblings. For example,
the sibling space in age, the number of siblings within the family and the
socioeconomic status may all vary depending on the gender of the first
child.
20
By comparing siblings in families in which one sibling has died with families in
which all the biological siblings have grown up together, the authors show that it is
the social order among the siblings rather than the birth order itself that matters
for explaining differences in IQ performance between the siblings.
