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With declining family size and a gradual shift in family size ideals, only child families are becoming more common in many societies. Although previous research shows that in terms of cognitive outcomes only children tend to do as well as children with few siblings and better than children from large families, other studies show that they experience a disadvantage. One potential explanation for the mixed results across studies is that they cover different time periods: if the socio-demographic composition of only child families changes over time, the outcomes of only children compared to children growing up with siblings could, as a consequence, also vary over time. This possibility remains largely untested in the literature. In this study, using data from four British birth cohorts (born in 1946, 1958, 1970, 2000-2002), we test whether the association between being an only child and cognitive ability at age 10/11has changed over time and, if so, whether it is explained by cross-cohort differences in the characteristics of only child families. The results show that only children have higher cognitive ability scores than children who grow up with siblings but also that the advantage has weakened over time. Adjustment by family socio-demographic characteristics attenuates within and cross cohort differences. Moreover, the results show that the cognitive advantages associated with being an only child vary considerably by whether or not the cohort member has been exposed to parental separation, and that being an only child does not attenuate the negative association between parental separation and cognitive ability. Taken together, the findings underline that the selected socio-demographic characteristics of only child families are critical in understanding the link between only childness and cognitive ability in childhood and how it varies over time and potentially across existing studies.


Alice is associate Professor in Demography and Deputy Research Director at the Centre for Longitudinal Studies located in the UCL Social Research Institute. Her research interests span a number of substantive areas in social demography and epidemiology. Her research has examined the association between advanced maternal age and child well-being, with a particular focus on whether and how it varies across different groups of the population and time periods. She is also interested in understanding the determinants of child obesity and other markers of health, including if they vary by ethnicity. More generally, she is interested in whether, and if so how,  family processes are associated with children and adults’ well-being. Her research has been published in demography, multidisciplinary and medical journals such as the Lancet, PNAS, Demography and the International Journal of Epidemiology.