A new working paper by three economists contains grim news for parents of many kids: Gaining siblings modestly reduces children’s scores on cognitive tests, worsens their behavior, increases their risk of criminal conviction and pregnancy as teens, and lowers their educational attainment and earnings in young adulthood. Yet members of large families might take heart from some lesser-known findings about family size that I’ll share below.

Analysis by the new paper’s coauthors—Chinhui Juhn, Yona Rubinstein, and C. Andrew Zuppann—suggests that the time shortage of parents with more children is one important factor behind their results. Having another child does not seem to deprive the family’s older child(ren) of parental affection or intellectually stimulating resources in the household, or cause the home to become unsafe or overly messy, but it does mean the older child(ren) will spend less enriching time with their parents, even beyond the new baby’s infant and toddler years.

Juhn et al. do not dwell on parental income in their paper; however, prior research indicates that the financial strain of having more kids is also relevant. For example, parents of larger families are less likely to pay for their children’s college education, and having a college degree and minimal student loan debt gives young adults a better start in life.

Juhn and her colleagues reached their conclusions after examining twenty-six years of detailed data on thousands of American mothers and children who participated in the National Longitudinal Survey of Youth 1979 and the associated Children and Young Adult Survey. Instead of directly comparing children from different family sizes to one another, they used three methods designed to control for possible confounding factors, such as looking at how individual children behaved and performed cognitively before and after their parents had another child.

How Much Does Family Size Matter?

On average and controlling for several demographic and background factors, the researchers find, “raising final family size by an additional child reduces average schooling of children by -0.13 years”—not a whole lot.

Their estimates based on the outcomes of older children in families with twins are more striking. By early adulthood, members of this group are 12.6 percentage points less likely to have graduated high school, 7.46 percentage points more likely to have been convicted of a crime, and 9.36 percentage points more likely to have experienced teen pregnancy than comparable young people from families without twins, and their educational attainment is 0.395 years lower. (Again, that’s controlling for background characteristics of mothers and children.)

Examining families with twins offers advantages and disadvantages. They have had a “plausibly exogenous increase in family size,” or an increase unrelated to the family’s other characteristics, meaning “differences in outcomes for older children in households with twins versus households without twins can thus be interpreted as a causal estimate of how family size affects outcomes,” per the paper. Having twins is relatively uncommon, however, so even in the large overall sample of the NLSY, there were only 142 children who had younger siblings who were twins, which the coauthors considered a drawback. And it’s possible that raising twins is harder for parents than having two additional kids who are spaced apart.

Other Factors

In addition, the exact effects of gaining a sibling on children depend on several other factors, such as their gender, their mother’s marital status, their mother’s score on the Armed Forces Qualification Test—a measure of math and verbal ability—and possibly their home country. The arrival of a new sibling has “large and significant” negative effects on girls’ cognitive scores, according to Juhn and her coauthors, but small and statistically insignificant effects on boys’ cognitive scores. The negative effect on behavioral problems, however, is larger for boys than for girls.

Less surprising is the significance of parents’ marital status. Children of mothers who are married (as of the survey wave prior to the birth of the new child) are less negatively affected by new siblings than children of single mothers, Juhn told me via email. Presumably that’s because two parents can usually offer more financial and time resources than one. Juhn added that “the effects are still there and strong for the married mothers,” meaning the findings are not all driven by single-parent families.

Mothers’ AFQT scores also shape the effect of family size on kids. As Juhn, Rubinstein, and Zuppann explain in their paper:

For children with mothers with below median AFQT scores, the arrival of younger siblings [has] large and significantly negative effects on cognitive skills while for children with mothers with above median score[s], the effects are much smaller and not significant. With regards to [the] “Behavioral Problems” index, however, the results are opposite. It is the children of high ability mothers who are more likely to act up when a younger sibling arrives.

The economists found that mothers with above-median AFQT scores cut their work hours more than other mothers after having an additional child, and believe this divergence could help explain the varying cognitive effects of new siblings on kids. They note, too, that mothers with higher AFQT scores likely have access to other resources such as better child care and more generous maternity leave, which should make it easier to support a larger family successfully.

Public Institutions and Community Norms

Earlier studies imply that the significance of family size varies from one country to another. Juhn et al. cite research showing that in India and China, as in the U.S., having more siblings lowers children’s educational attainment, whereas in Israel and Norway, there is no negative association between family size and schooling when careful statistical techniques are used. The coauthors believe these conflicting findings could result from differences in countries’ public safety nets and education systems. Family size may be relatively important in the U.S. because of our relatively weak safety net and public education system.

Other research hints at yet another factor determining the importance of family size: the norms and social integration of a family’s community. Douglas Downey, a sociologist at Ohio State University, reports the following:

Studies reporting weak relationships between [number of siblings] and educational outcomes are typically ones that have studied individuals from highly integrated communities with norms supporting large families [such as Mormons in the U.S. and Muslim Arabs in Israel]. One reason for this pattern may be that a child in this type of community has a larger group of adults nearby who have an interest in the child’s well-being—aunts, uncles, older cousins, grandparents, and other adults—and this feature buffers the dilution process occurring within the nuclear family.

The factors influencing the magnitude of the negative effects of family size suggest several ways (albeit not easy ones) to mitigate them. Ensuring that all families, not only those with high incomes and cognitive abilities, have access to good schools and beneficial work-family policies would be one place to start. Helping couples with children to get and stay married, and involving extended family and community members in caring for children, may also help.

The Benefits of Big Families

Despite the disadvantages documented above, having siblings appears to help children in some ways. Kids with siblings have greater social skills than those without, and married people who grew up with more siblings are less likely to divorce, controlling for many background factors. One can imagine other tradeoffs as well. Only children may benefit from their parents’ undivided time and attention in childhood, but wish in adulthood that they had siblings with whom to share the responsibility of caring for their aging parents. Kids in big families might spend their childhood bickering and competing with their siblings, but form close relationships with them in adulthood—or vice versa.

Finally, the latest published research indicates that the effect of siblings on social skills, at least, hinges on the quality of sibling relationships. In short, the benefits and drawbacks of different family sizes vary between families and over the course of a lifetime, in ways that parents can’t always predict.