And that, ladies and gentlemen, is a huge, scary treatment effect

We exploit the occurrence of postpartum depression (PPD), which has a random component according to the medical community, to estimate causal effects of a salient form of mental illness on couples’ relationship status.

…We find that this relatively prevalent mental illness reduces the probability the couples are married (by 22–24%) as well the probability that they are living together (married or cohabiting) (by 24–26%) three years after the birth of the child.

Full paper.