A new study in the journal PLOS ONE asks whether fathers who sleep in the same place as their children have lower testosterone—and finds a significant link.
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.