We present a new theory and quantitative evidence on the incidence of unigeniture. Existing theories hypothesize some sort of indivisibility or increasing returns to scale in wealth to overcome the testator’s inclination toward equal sharing among heirs. Examining a micro-level data set of seventeenth-century English wills, we find that even after controlling for land and other presumed sources of increasing returns to scale and indivisibility, other variables such as financial wealth still correlate with the incidence of unigeniture. This leads us to propose a model where unigeniture is an efficient way to provide insurance against income shocks. An important implication of the theory is that observed unequal bequests are partially counteracted by ex post transfers. Therefore, inequality of bequests cannot be taken as evidence of inequality of consumption between heirs.