A recent study that found that parents exposed to cold viruses are less than half as likely to become ill as nonparents runs counter to the experience of many moms and dads with their little germ factories — uh, darling children.

The study's leader, Sheldon Cohen, a professor of psychology at Carnegie Mellon University in Pittsburgh, has spent most of his career studying the extent to which psychological stress, social support networks and social status influence our immune systems and susceptibility to infectious disease.

Although he has no children himself, Cohen and his team correctly surmised that "being a parent can be stressful" and so regarded that as a risk factor along with marital and employment status, education, etc. On the flip side, the researchers also considered that being a parent could provide a sense of purpose and help establish supportive social networks.

Then, as reported in an upcoming issue of the journal Psychosomatic Medicine, they exposed 795 adults between the ages of 18 and 55 to a common cold virus and kept them quarantined for six days to see who got sick. And they took into account any innate immunity parents had picked up because they lived around kids.

So, parents were 52 percent less likely to develop a cold than nonparents. But not all parents were equal: The parent effect only kicked in for people older than 23. Cohen figures that's because being a young parent, with young kids, may be extra stressful and does not bring many social support advantages.

Parents with three or more kids were 61 percent less likely to get a cold than nonparents. But the biggest advantage fell to older parents whose children no longer live at home. They were 73 percent less likely to get sick from the virus.

Cohen and his colleagues admit that while it was clear parenthood was protective, "we were unable to identify an explanation for this association." They simply didn't measure any psychological or biological differences between the parents and nonparents that could account for the different reactions, but they insist there must yet be some psychological benefit of parenthood that let so many more shrug off the viral assault.

With all respect to the learned researchers, those of us who have stocked the tissue boxes, filled the vaporizers and mopped up after sick kids over the years can surmise our own reason: We earned it fair and square. If we picked up extra immunity along the way, isn't that the least Mother Nature can do for us?