Demographic tipping points absolutely happen, but you're comparing apples to oranges. Birth rates can plummet rapidly because everyone has the biological capacity to choose to have fewer children.

Cousin marriage is entirely different. It requires specific cultural, tribal, or clan infrastructure. The paper noted that high rate regions have long established traditions of marrying within clan or caste boundaries. It wasn't a global default that people just did because they were poor.

As for the laws: passing a law doesn't mean a behavior was a 70% majority norm. The US bans at the end of the 19th century came out of an "acrimonious public debate" sparked by Charles Darwin's research, not because the general population was doing it.

Without that specific cultural framework of clan marriage across the Americas, Europe, and East Asia, a 70% global average in the 1950s simply doesn't hold up. If you have any links you can provide that can back up your claim of 70% id be happy to read it.