Why is that in almost all known societies, men are on average older than their female intimate partner? And how – and why – is it that the average age gap between spouses or intimate partners varies by degree of socio-economic development (over time as well as among societies) and, among couples, by age at which the two people became a couple, as well as man’s social position? The international empirical literature on the question is reviewed, and the explanatory model indicating that individuals may well have “good reasons” for forming couples in which the man is older than the woman is examined. An empirical test of this model bearing on couples formed in France between 1978 and 1998 suggests that the model’s explanatory power is good, though in itself it does not explain the reversed social gradient of the age gap that occurred in the second half of the twentieth century, a phenomenon probably due to the prolonging of men’s education.