The study follows another by the same group that found that fatherhood itself decreases testosterone levels.
It's unclear to me whether the new study shows that cosleeping causes low testosterone, or whether low testosterone could cause fathers to be more willing to sleep next to their children. The researchers say that the study finds "that close sleep proximity between fathers and their offspring results in greater longitudinal decreases in T," which seems to suggest that cosleeping causes a drop in low testosterone, but it's not the strongest statement I could imagine.
Still, it's an interesting finding, in the context of other studies that have associated low testosterone with fathers who are more nurturing with their children. The studies suggest that a fall in testosterone is associated with the change from competing with other males for mates to making a parental investment in children.
One of the problems, as the researchers note, is that it's unclear how cosleeping could alter testosterone levels. Further, the fathers' testosterone was reduced in the evening but not in the morning—another finding that's difficult to explain.
The study was done by Lee Gettler and colleagues at Notre Dame and by researchers in the Philippines. The subjects in the study were 362 men from the Philippines who were part of a long-term effort to monitor testosterone levels and behavior.
There is surely more to learn and more to say about this study. The link between testosterone levels and child-rearing by fathers is an important one, and understanding it could go a long way to explain why human fathers are among the very few males who stick around to raise their children.