within-siblings variation is very long and covers many fields in economics (see e.g. Krashinsky, 2008) measures family effects on voting preferences). However, when applied to siblings rather than twins, one should recognize that the idiosyncratic component which cannot be removed may be relevant. Griliches (1979) and Neumark (1999) caution that neglecting the individual component might induce a distortion in within-siblings estimates even larger than in pooled OLS estimates. To limit this problem we have included among the regressors the order of births of the siblings, which in combination with age should capture differential allocations of resources within the family as well as differential expositions to the family background. In Section 6 we specifically address this concern more in detail, by approximating the ideal case of twins by restricting the sample to the siblings whose age difference is at most two or three years.

We have extracted from the EHHUES2002, which counts about 27,000 observations of people aged 16-64 (working age according to Egyptian regulations), the subsample of all heads’ sons and daughters residing in each household from which we have removed the only sons. The resulting sample, labeled “sons”, is then composed of at least two siblings per household. Furthermore, we have extracted from the full sample the sample composed of household heads and of their own brothers, exploiting the detailed information on household members’ relationships. In Egypt, as in many middle-income countries, it is rather common to find extended families, where several generations live together. We label the resulting sample “head/bros”. The union of “sons” and “head/bros” produces the sample “siblings” that we have used throughout the paper. In Table 1 we have reported some key summary statistics referred to “siblings”, to its component subsets, to the full sample of the aged 16-64 and to an additional sample composed of the “only sons”. The latter is the sample of heads’ sons who live with the household heads and have no other brother or sister living with them. This does not mean that they have no brothers at all, but only that their brothers possibly moved out from the original household.

We are interested in investigating whether the sample of siblings can be representative of the general population or whether the choice of siblings introduces serious self-selection bias. While selection on observables is not problematic, as it will be accounted for by the included controls, our concern is that the sample selection depends on unobservable characteristics. For instance, it could be the case that less endowed sons prefer to remain with their parents and brothers, in order to receive support and benefit from their family. In this case unobservable individual endowment will determine both the selection and the performance on the labor market. Fortunately, the within-siblings estimator, by removing a large share of individual unobservables, is quite robust to sample selection, as we shall discuss in detail in appendix B. In particular it is fully robust, if all siblings share the same endowment, i.e. if for all _i_ in household _j_ . Otherwise, the interpretation of the within-siblings estimates is that of marginal effects in a Heckman-type model.

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