But a new study shows there are convincing reasons not to romanticize large families. A recently published paper from three economists that looks at 26 years of data on parents and children suggests that with every additional kid born, the other siblings are more likely to suffer from lower cognitive abilities and more behavioral issues, and have worse outcomes later in life.

The paper builds on older research that claims that families face a trade-off between the quantity of kids they have and the "quality" of each kid -- an awkward term that refers to things like how much education the child receives, whether they are employed when they grow up, and whether they end up with a criminal record. The research also supports now-popular ideas about early childhood development, that the time and resources that parents devote to young kids have lifelong impacts.

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The economists -- Chinhui Juhn, Yona Rubinstein and C. Andrew Zuppann -- drew on a large data set from the National Longitudinal Survey of Youth in 1979, which asked families a ton of questions about their children's math and reading abilities, behavioral issues, and home environments -- such as how often parents read to children or help them with their homework.

The researchers analyzed how older siblings performed before and after a younger sibling was born. They found that their measure of parental investment in older kids -- represented by things like how often families eat meals together, how often parents show kids affection and how many books each child has -- fell by 3 percentage points after a young child is born, while cognitive scores fell by 2.8 percentage points and behavioral problems increased.

The effects also differed depending on the older kid's gender, Juhn and her co-authors found. Girls who gained younger siblings were more likely to see their reading and math scores suffer, while boys were more likely to have behavioral issues.

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The birth of another kid also lowered the overall amount of education that each child received. For each additional child born, the researchers found that the average amount of education the kids in the family received fell by -.13 years. And older children whose parents had twins ended up with nearly half a year less of education than other families.

The research showed that these effects weren’t a temporary side effect of the birth of a younger sibling, but persisted through childhood and into later life -- resulting in lower education, lower earnings, more criminal behavior and more teenage pregnancies.

The graphs below show the effect of an additional child on a first-born sibling for the years before and after the younger child is born. The first chart shows a decline in the "HOME Inventory Index," which is their measure of parental investment in a child. The second chart shows the decrease in cognitive ability, measured by things like math and reading scores, while the third shows the increase in behavioral issues.

To figure out why exactly these trends were occurring, the researchers split up "parental investment" into four different factors -- the time parents spent with their kids, the resources (including money, books, clothes, etc.) they devoted to each child, the affection parents gave their kids, and how safe the home environment was -- and investigated each.

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They found that the amount of time that parents spend with each kid is the factor that changed the most following the birth of another kid. The other factors didn't change significantly, they said.

Inequality in everything

Juhn, Rubinstein and Zuppann did find one external factor that appeared to have a big influence on these outcomes -- the cognitive abilities of the mothers.

Mothers in the original data set were also made to take the Armed Force Qualification Test, which is a test the military uses to assess reading, reasoning and other basic abilities. The researchers found that children of mothers who didn't score well on this test saw a much larger drop in cognitive skills when another child arrived.

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One plausible explanation is that mothers with higher scores tend to be better educated and wealthier. Indeed, mothers with higher scores were more likely to quit their jobs or reduce the hours they worked after having another kid, while mothers with lower scores were likely to work the same amount -- perhaps because they didn't have the option of quitting their jobs. Compared to those with lower scores, mothers with the high scores, and presumably more education, appeared to face less of a trade-off between the number of kids they had and how much time and resources they could devote to each.

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The researchers say this could be partly due to a lack of supportive policies in the United States for mothers and families -- including parental leave and child-care coverage. This could explain why economists have seen a so-called "quantity-quality trade-off" in some countries, but not in others, they write.

For example, one study published in 2005 that looked at families and kids among the entire population of Norway turned up substantially different findings. That study showed only negligible effects from family size on a kid's performance. However, a similar study done in China showed a big effect, with kids in larger families receiving much less schooling, especially in rural areas.

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Juhn and her co-authors hypothesize that the discrepancy could be due to different policies in different countries.

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Countries like Norway, which have extensive welfare programs, parental leave and other supportive policies for families, and strong public education, might see less of a trade-off for families between the quantity and quality of children, they hypothesize. But parents in countries like the United States and China, which have a more limited support system for families and lower-income people, might be forced to choose, they say.