The time has come for "Hey, Science," America's only popular science column, in which we enlist real live scientific experts to answer humanity's most provocative/ dumb scientific questions. Today: does your own baby's poop smell better than the poop of other people's kids?

Today's question, which was selected in response to a slew of anecdotes from parents employed at Gawker Media: MANY parents report noticing that their own child's poop smells somehow better (or less bad) than the poop of other kids. This is obviously a good thing, when it comes to changing diapers. But is this just wishful thinking on the part of parents, or a real and recognized medical phenomenon? Is it a psychological defense mechanism? A biologically rooted bond? What explains it?

Thomas DeWitt, director of general and community pediatrics, Cincinnati Children's Hospital:

I'm responding to you as a general pediatrician who is not an expert on smell and cognitive function, but as someone who did research on diarrhea many years ago and have had numerous discussions with colleagues and parents on the subject. I also would note that I tried to steer clear of the subject at dinner tables. Clearly an off-beat question that I am sure may be a topic of discussion in non-academic circles more often than we realize. I did a brief search for any literature/data on the subject and came up relatively empty. I did find one article from 1995 that examined the impact that odor had on people's perceptions of other individuals. (J.Todrank, Learning and Motivation 26, 116-140, 1995). There, not surprisingly, was a negative correlation (or positive, depending on your point of view) – a "smelly" odor negatively affected the perception of the person. I mention this article because I would suspect that the reverse may be functioning – a positive view of an individual, in this case an infant, may affect the perception of the odor of the stool. There are several factors that may affect the odor of the stool – primarily determined by the bacterial content of the intestines (mostly in the large intestines) and the substances that they have access to – i.e. what the baby eats. Stools from breastfed babies tend to smell differently from formula fed babies. In infectious situations, stools from viral diarrhea tends to smell differently from bacterially caused diarrhea. Some people have suggested that pheromones may be a factor, but human pheromones tend to come more from the skin, and maybe vagina, than from stool. Another possibility is that the infant has a bacterial mix in the intestines that has come from the mother, and the stool smells similarly to her stool – addressing the concept that one's own stool doesn't smell as bad as other stools. Whether this has any evolutionary benefit would be even greater speculation than the preceding comments.

Hope these comments are some help…..don't think that you will get an "evidence-based" answer.

Karen Dougherty, pediatrician and educator:

A very good treatment of the subject is featured in a Salon book review, which you can see here.

[The most relevant portion: "People don't exactly know how this works, but acute exposure to something can have the effect of decreasing our feeling of disgust toward it... [an] example is that over time, mothers become less disgusted by the dirty diapers of their own child, but they remain disgusted by the dirty diapers of other peoples' children. But what's happening there isn't conscious. It's automatic. In general, there's not a lot known about the ways we can deliberately or voluntarily make ourselves not be disgusted by things."]

First, pediatricians do hear a LOT of discussion of baby poop...some days it seems like the only topic: too much, too little, weird color, etc. One consistent comment concerns the marked changes in scent that come as new foods are introduced, especially meat. When babies change from breast milk only, which produces stools with a distinctly cheesy or sour (but not typically "poopy") smell, parents are horrified. That's yet another reason for parents who are ardently breast-milk-only to feel smug—their kid's poop doesn't stink.

Probably, though, the phenomenon is a mix of evolutionary biology and psychology. The smell of feces is universally repugnant, because if you eat poop you increase the odds of getting sick and dying. We are the children of ancestors who said, "I think I'll pass on that, thanks." However, many popular foods have distinctly fecal overtones. Think stinky cheese or even some coffee. Because we learn to associate these foods with yumminess, we ignore those elements, at least until a fellow diner crassly points them out. So if your baby's poop comes with other pleasant sensory experiences—-baby lotion, cuddliness, smiles, etc.—-that smell becomes part of the whole I-Love-This-Kid experience.

A simpler explanation involves simple habituation. Anything that's awful that you deal with all the time becomes less awful. We dissect icky rats and fetal pigs in my anatomy classes. When students first begin, the smell and general ickiness of the experience is off-putting, but as we proceed virtually everyone—-even the real rodent haters—-is able to overcome the disgusting aspects. Students take pictures to show their families, who are typically horrified by what mom or dad did at school today. So, a parent who deals daily with his/her darling's diapers will become less sensitive to the noxiousness of that odor.

The verdict: Though the phenomenon of your own child's poop seeming more appetizing is well known to science, much remains unknown about the psychological and physiological implications of baby poop. The most likely causes of the phenomenon would seem to be positive psychological associations with your child and/ or habituation, which cause you to develop more a tolerance for their particular brand of poop. There may also be biological factors, such as similarity of gut flora between you and your child.

We can also now say that it is very hard to find scientists to answer poop-related questions. Our eternal gratitude goes to those brave enough to do so.


The full archives of "Hey, Science" can be found here.

[Image by Jim Cooke. If you have a question for "Hey, Science," email]