Sibling Similarity in Family Formation

Sibling data have been widely used to analyze the impact of family background on status attainment. To a lesser extent, they have been utilized for examining family-of-origin effects on demographic outcomes, such as leaving the parental home, union formation, and fertility. This research gap is surprising in view of the increased interest in the interdependencies between demographic processes among family members and their role in the (re‑)production of social inequality across generations. We address this issue in our recently published paper on sibling similarities in family formation.

Our starting point is an explorative comparison of similarities in family formation among siblings and among comparable unrelated persons. If demographic behavior is influenced by family background, siblings should be more similar in their family formation than unrelated persons.

For the analysis, we used Finnish register data from 1987 through 2007 and constructed longitudinal family-formation trajectories in young adulthood from ages 18 to 30 for siblings and unrelated dyads (N = 14,257 dyads). The dyadic family formation sequences follow two dyad members – siblings or unrelated individuals – family formation trajectories in terms of living arrangements, partnership status and number of children. Figure 1 shows an example of a similar dyadic family formation sequence (dyad 1) and a dyad in which difference between the two dyad members’ family formation sequences is more pronounced (dyad 2). Findings from sequence analysis show that these family formation processes are indeed moderately but significantly more similar for siblings than for unrelated dyads. The similarity effect is particularly pronounced among same sex siblings, i.e. sisters and brothers.

Siblings naturally share some commonly observed parental background characteristics, such as parental education and early childhood family structure that unrelated persons usually do not share. Additionally, siblings share a unique and typically unmeasured experience of growing up in the same family environment. These compositional effects alone might generate sibling similarity in family formation, also in the fairly egalitarian Finnish welfare state with supposedly modest family of origin effects.

Therefore, we went a step further and asked to what extent observed shared parental background characteristics account for sibling similarity in family formation. More specifically, we emulated an experimental design by comparing sibling dyads with a control group of unrelated dyads that are identical with regard to important parental background characteristics, i.e. parental education and early childhood family structure. Figure 2 illustrates the analytical approach. In contrast to the first comparison, unrelated dyads should now be more similar in their family formation if shared and observed parental background accounts for sibling similarity. If the effect of shared parental background characteristics is very strong, unrelated dyads might even become just as similar in their family formation as sibling dyads.

Contrary to these expectations, the results revealed a surprisingly small effect: equalizing siblings’ and unrelated dyads’ parental background characteristics decreases the sibling effect only by a modest 13 percent. This does not mean, however, that social origins do not matter. Family formation is indeed affected by parental education and parents’ marital history but this is true for both siblings and unrelated dyads.

In addition, the results revealed that gender and the children’s own education are more decisive forces in explaining similarities in family biographies. These factors also proved to be more relevant in illuminating in what way siblings’ family formation is more similar than the family trajectories of unrelated dyads.

To gain a deeper insight into the nature of this sibling similarity, we examined whether siblings have a higher probability of experiencing the same substantive family-formation patterns than unrelated dyads. We found that patterns corresponding to economic disadvantage, e.g. family trajectories with early parenthood out of wedlock or high fertility, are indeed concentrated within families. This is in line with a growing body of research highlighting the importance of family structure in the reproduction of social inequality.

The fact that Finland represents a fairly egalitarian welfare state with supposedly small family-of-origin effects suggests that these effects might be stronger in other social contexts.

Figure 1 – Example of two dyadic family formation sequence



Figure 2 – Illustration of research design


This post has been jointly written by Marcel RaabAnette Eva Fasang, Professor of Sociology at Humboldt-University Berlin and Head of the research group Demography and Inequality at the WZB Berlin Social Science Center, Aleksi Karhula, doctoral candidate at the University of Turku, and Jani Erola, Professor of Sociology at the University of Turku, PI of ERC Consolidator Grant Project “Intergenerational Cumulative Disadvantage and Resource Compensation” and Editor of Acta Sociologica-journal.


Raab, M., Fasang, A., Karhula, A., & Erola, J. (2014). Sibling Similarity in Family Formation. Demography, 51 (6), 2127–2154. doi:10.1007/s13524-014-0341-6.

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