Recent studies have shown that cousin marriage was rare among ancient humans, with only 3% of almost 1800 humans going back about 45,000 years showing any signs of cousin marriage. This may be due to increased genetic diversity that can prevent genetic mishaps from occurring.
Historically, marriage between first cousins in China was generally acceptable, except for the kids of two male siblings. However, the Catholic Church later imposed the rule that no one could marry anyone closer than their third cousin, though the reasoning behind this is not entirely clear.
Building trust within a society was crucial for expanding beyond kin networks and forming larger groups. Traditions such as questioning potential familial ties between wedding couples evolved into speech asking for objections to the marriage, which were rooted in the necessity of human trust within non-related groups.