I agree with you, bijou. I think I have heard of a couple of instances in lower income communities of half siblings creating children together because the lines of lineage are so precarious with the various mixings of dna, with several half siblings being within a block or two of one another.
That does happen, people often think that it is a rural problem because of the isolation. But it happens when people don't want to marry out of their small ethnic group or they are 'isolated' by living in a ghetto of sorts and not interacting outside of a relatively small range of people. So it is a problem for cities too.
Yes, and an urban area is precisely what I was thinking of. I can remember places like Goulds in Miami where I'm fairly certain many residents of the community did not leave that immediate area much. I do remember some talk as well of a few of the couplings that were relatives unknowningly. The matriarchs usually try to keep a handle on whose child is whose I think more for a matter that the children do know who their siblings are in the interest of relationships, but there are many children that are products of cheating out of the primary relationship that don't get the attention. And there are MANY of those relationships out of the primary one.
What I'm about to say is only speculation and my own observation of course, but this community had very high rates of asthma and some birth defects. At the time, I thought perhaps(like the good little liberal I was) that it was due to their living conditions. Anymore, I have to wonder if it is not unknown couplings of half siblings and cousins that are creating genetic problems for their offspring.