Hereditary Dislike of Family

The extensive genealogy work my spouse has done has barely penetrated one branch of my family, and it's not because it lacks descendants! Unlike the other relatives, the ones in this branch admit to knowing little or nothing about their parents' and grandparents' generations. Many in this branch have not responded to our inquiries; two have told us to never contact them again.

So why does antipathy towards relatives seem to be concentrated in this line?  An interesting idea is to consider the evolutionary consequences of a heritable dislike-your-family trait.  Such a trait seems present in the life cycles of some social species where only a minority of adults leave their clans.  While this trait would, on average, probably reduce the survivability of the individuals expressing it, that could be outweighed by the increased dispersion of the clan's genes.

Such a trait could have more than one cause.  One possibility is the types of mental illnesses which often result in the breakup of families.  Schizophrenia in particular tends to manifest in late adolescence, the time when individuals become fertile and able to live independently.  Separation from the clan at that time maximizes their chances of mixing their genes with other clans in offspring.

If mental illness benefits the species by increasing genetic diversity (which is evidenced by its global occurrence), then mental illness is an integral and persistant part of the human genome.