According to the heterogamy hypothesis, social differences between spouses (in terms of age, religion, education, occupation, social origin...) bring about a higher level of marital instability (i.e. a higher risk to separate or divorce). In this paper we discuss the main points of this hypothesis and the empirical results that seem to support it. Our analyses suggest to replace the heterogamy hypothesis with a completely different one: we can find "frail" relationships where spouses do not respect the social rules regulating the formation of marital unions. In western contemporary societies these rules require the equivalence between partners or the superiority of the husbands. Couples where the wives prevail on their husbands live out of social regulations. For this reason they can suffer of higher risks of marital instability.