as a result. Having an older sister rather than an older brother can have a causal impact on young children’s development even in settings where older children do not contribute to human capital accumulation in their younger siblings. If older siblings’ investments in young children’s human capital are not productive, any treatment effect of having an older sister must be driven by parental investments: parents would invest more in young children when they have an older girl rather than an older boy because they have less incentive to invest in their older child. In contrast, when older siblings contribute to the development of young children’s human capital, parents with an older girl may actually invest _less_ time in their young children — because they have an effective substitute. Our model illustrates that any treatment effect of older sisters (on child development) that is not explained by differential investments by parents can be attributed to — and provides evidence of — the contributions of older siblings.

To test the empirical predictions of our model, we estimate the impact of older sisters on early childhood development in a sample of rural Kenyan households that have one or two young children aged three to six and exactly one older sibling aged seven to 14.<sup>2</sup> In this sample, we show that the gender of the older sibling is unrelated to household or community characteristics — and hence plausibly exogenous.<sup>3</sup> We find that young children with one older sister experience significantly more cognitive stimulation than those with one older 2 Reviewing ethnographic evidence from 50 traditional societies, Rogoff _et al_ . (1975) report that the typical age at which societies begin assigning older children childcare responsibilities is between five and seven years old. Ominde (1952) reports that in an area near Kisumu, Kenya, the “school-going age for the Luo girl” was the “age to which society has assigned the duty of nursing,” with the girls’ interest in this responsibility peaking at age “eight to nine years.” Capen (1998) records that the term _japidi_ is used for a “girl who cares for a child,” “nurse,” or “nanny.” Nearby, (Weisner _et al_ . 1977) report that (Kenyan) Luhya girls aged 6–8 years old spent 60 percent of their waking hours looking after younger children, though this caretaking was often under the explicit or implicit supervision of nearby adults. Similarly-aged boys (6–8 years old) and younger girls (aged 3–5) spent about half as much time caring for small children. Apoko (1967) and Lijembe (1967) also relate how in neighboring Acholi communities (where the role is called the _lapidi_ ) as well as in Idakho communities, young girls are usually tasked with caring for infants; if there is no appropriately-aged older sister, the task may fall to an older brother.

> 3An extensive literature treats the sex composition of children as a source of exogenous variation (cf. Angrist and Evans 1998, Washington 2008, Glynn and Sen 2015). However, the assumptions required for such estimates to identify causal impacts are unlikely to hold in general (Bisbee, Dehejia, Pop-Eleches and Samii 2017, Clarke 2018). In the United States, Dahl and Moretti (2008) show that having a firstborn daughter increases the likelihood of parental separation. In India, existing evidence suggests that son preference influences birth spacing and total fertility (Clark 2000, Jayachandran and Kuziemko 2011), so households with a firstborn son may not be comparable to households with a firstborn daughter.

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