The spacing between siblings doesn’t matter

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By: Kieron Barclay & Martin Kolk When is the best time to have another baby? This is a question that might intrigue parents planning to have more kids. But does it really matter for long-term outcomes in Sweden? In our new study, we find – in contrast to previous research – that birth spacing actually does not matter for how long people spend in education, how their cognitive ability develops or how they perform in the labour market. Here is why.

There are several good reasons to think that birth spacing should matter for long-term outcomes. These can broadly be grouped into biological explanations and social explanations. Being born after a very short birth interval might be bad for the child because his or her mother did not have enough time to fully recover from the previous pregnancy, and could be expected to increase the risk of poor birth outcomes such as preterm birth and low birth weight. Short birth spacing might also imply that the mother did not breastfeed, or at least did not breastfeed for very long. Since breastfeeding is associated with long-term benefits, missing out on this could be disadvantageous. Having closely spaced siblings might also mean that the resources and time that parents have available to invest in each child is diluted.

In a recently published study, we examined whether the length of spacing between births is related to long-term cognitive, educational, and socioeconomic outcomes. We wanted to investigate whether the length of time between births (birth-to-birth intervals) is related to how cognitive ability develops, how people perform in school, how long they spend in education, and how they perform in the labour market. Previous research has shown that short birth intervals (less than 2 years), and longer birth intervals (greater than 5 years), are associated with an increased risk of pre-term birth and low birth weight. Other studies have shown that short birth intervals are also associated with worse performance in high school, and a lower probability of going to university.

In our study we used data on the full Swedish population taken from government administrative registers. This resource allows us to link individuals to their parents and siblings, and to follow those same individuals from birth through the educational system and into the labour market. The specific outcomes that we examined were high school grade point average (GPA), IQ, the total number of years spent in education by age 30, earnings around age 30, being unemployed around age 30, and receiving government welfare support around age 30. We looked at people born 1960-1990 and examined how the length of time since the birth of an older sibling, as well as the length of time until the birth of a younger sibling, affected these long-term outcomes.

We also implemented an innovation in the way that we analysed the data: we compared biological siblings born to the same mother and father. This is important, because siblings in the same family share many similar conditions and experiences, such as living in the same house, and, critically, having the same parents. We also adjusted for several other factors that might be important, such as gender, the age of the mother at the time of birth, family size, birth order, and birth year.

In contrast to previous research, we find that birth spacing does not matter for long-term outcomes in Sweden. This is true for educational attainment, as well as for all of the other outcomes that we examined. We believe that the previous observed relationship between short and long birth spacing and poor outcomes is related to concentrated disadvantage. Short and long birth intervals seem to be more heavily concentrated in families that are already disadvantaged in terms of health and socioeconomic status. Since children from these disadvantaged families are less likely to do well in the educational system and labour market, this explains why short and long birth spacing are superficially associated with worse outcomes in adulthood.

Sweden is a highly developed country with excellent medical care, extremely low rates of infant mortality, relatively low income inequality, and free education. Naturally, therefore, the effects of birth spacing might be more consequential in a country where the residents do not benefit from these advantages.

Read the full article (open access) here: Barclay Kieron J. and Kolk Martin (2017) The Long-Term Cognitive and Socioeconomic Consequences of Birth Intervals: A Within-Family Sibling Comparison Using Swedish Register Data, Demography, Volume 54, Issue 2, pp 459–484

A previous version of this blog post was originally posted here, in IUSSP’s Online Magazine.

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