On the other hand, the majority of related studies seem to be observational, rather than interventional, so it's quite possible that both co-sleeping and observed "effects" are the result of some third factor, such as the attitude of the parent. For example, it's likely that a parent who chooses to co-sleep is more well-disposed toward the infant, and is therefore far less likely to kill it deliberately (infanticide), thus making up some unknown decrease in the overall frequency of "SIDS".