For 72 countries, I calculate a correlation of .44 between the mean IQ of a country and the rate of divorce. The greater wealth of intelligent countries reduces the economic necessity of marriage, but the correlation between per capita GDP and divorce is only .28.
I suspect that less intelligent countries are more likely to see marriage as a change in life status, not a contract. The acid of modern rationality reduces institutions like marriage to voluntary agreements that can be dissolved if unsatisfying. The focus on family used to be intergenerational; now it's business between adults.