It's possible that some small part of the reason that some single parent families don't tend to do as well as kids from homes not "broken" is because like other animals we tend to pick on the weak, including those with weak social ties. Imagine hyenas (social creatures much like ourselves) with many of the younger ones having mothers and fathers in the pack, would be at a large advantage to those with no direct relatives in the pack. But unlike hyenas, the male of the species is the one with most of the physical influence. It's obvious to me that at least some small part of why children with no fathers do worse is because they are an easier target - a safer target to pick on. As social creatures, we have pecking orders much like other animals, and we are constantly re-evaluating our places in these pecking orders. As a consequence of evolution, we sometimes, even in today's modern world, pick on the socially weak to prop ourselves up in the pecking orders. Behavior is very complex, but I have no doubt that there is some truth to this.
The answer regards children in single parent families appearing to fair worse than others is very complex, one reason is as seen below: Males (regardless of what single mums like to say) do NEED positive male role models, now in some cases, this can be grandfather, uncle, close family friend, lover etc, and so the position is filled, but in situations where there are no postive male role models, when boys reach puberty and they are seacrhing for their 'identity' and someone to identify with, there is a problem if this role model is lacking. They may then turn to TV role models, or their peer group, perhaps older boys. These role models may NOT be positive, may be gang orientated, perhaps made up of other boys also without role models and so they try to find their own way and define what they think (as they really do not know) what defines a 'man'. Is it bravado, violnece, crime? There is an inner struggle. Either that or they may instead identify with the positive females in their family unit and demosntrate more feminine qualities (not making them gay!) as a result. This is my opinion based on what I have seen from experience. Myself as a female raised by positive females (in single family) the only negative was that I did not 'learn' how to flirt from flirting with my father, which all little girls do. So I had to 'learn' this later in life. I also grew up more independant as a result, is this a negative? I am however now very traditional lover and mum, so I did not as a result turn into a raging femenist either. Single parents can try to avoid these problems with young men if they seek to maintain relations with positive male role models, perhaps enrole the boys in cubs/scouts. Army cadets etc if there are no male family/friend ties. Back to your point, I know of no one picked on for being in a single family and that includes myself and my brother. It was an issue that never arose. Some single mums may gain themselves a bad name but that depends on how they present themselves.
It is because society has absolved itself of any responsibility for each other's children. It isn't the only method of social organization, in some societies, children are thought to be the responsibility of everyone, and it doesn't matter wether the biological father is involved. The biological mother will probably always be important, since they bonded in the child's early years.
