Some smells automatically bring a smile to our faces: Puppy breath. New car smell. The aroma of fresh-baked cookies.
For moms, it's the scent of their babies. Research shows most moms find the smell of their bundle of joy irresistible, while babies find their moms' odor unique — one more way nature strengthens a bond that assures survival of the species. In fact, 90% of new moms can pick out their baby by smell within 10 minutes to an hour after birth.
But do we continue to like the smell of our children as they age?
Not so much, a new study finds, especially when those children are teenagers in or past puberty.
"This has something to do with the changed composition of the infantile sweat due to the increased release of sexual hormones," said professor Ilona Croy, who studies the sense of smell at the Dresden University of Technology in Germany.
The sweet smell of babies
It's possible that all women are evolutionarily wired to respond favorably to "new baby smell."
A 2013 study found the reward centers of the brain lit up in a small group of new moms and women who had never given birth when they smelled pajamas that newborn infants had worn for two nights. The PJs had been frozen and were presented to the women some six weeks later. None of the women were related to the babies.
Of course, the bonding benefits are not available to parents who cannot smell, either due to a physical or psychological issue.
"We did a study where we could show that mothers who have — because of various mental disorders — problems bonding with their child, show an abnormal olfactory perception," said Croy.
Ordinarily "mothers prefer the odor of their children before the odor of others," she added. "Those mothers are neither able to identify their child, nor do they prefer it."
The new study blindfolded 164 German mothers and asked them to smell body odor on clothing from their own child and four unfamiliar, sex-matched children. Clothing samples consisted of onesies that infants had slept in for one night, or cotton T-shirts slept in for one night by kids up to age 18.
Moms accurately picked out a strange child's developmental level from the smell 64% of the time; success rate was even higher when the child was their own.
Mothers also scored higher when identifying odors in children who had not yet hit puberty, and found those much more pleasant — "sweet" was the most common response, said Croy, who supervised the study.
The 'stinky' smell of teens
Stronger, "high intensity" body odor samples were identified as coming from children in, or past, puberty.
"Body odor is perceived more intensively due to the developmental changes," explained lead author Laura Schäfer, a doctoral student in Croy's lab. "Pleasantness and intensity perception are often negatively related."
In fact, moms got it wrong if an older child past puberty had a "pleasant" smell, classifying those odors as coming from a younger, pre-pubescent child.
Schäfer said the study may be the first to investigate whether parents can determine a child's developmental maturity by smell. Putting this in context with prior research, she said, the implications for parent-child bonding as children grow could be significant.
"Many parents report that their baby's odor smells pleasant, rewarding and adorable," Schäfer said. "This suggests infantile body odors can mediate affectionate love towards the child in the crucial periods of bonding.
"This seems to decline with increasing age," she added, which could be interpreted as a "mechanism for detachment, when the child becomes more independent and separates itself from parental care."
So perhaps we are evolutionarily supposed to find our children stinky as they age, so we'll let go and allow them to become independent?
"Smelling can be an unconscious factor that can influence perception and thus also the relationship," Schäfer said, adding that parents shouldn't be "irritated if they do not find the smell of their own child in puberty very pleasant."
"It is important to note, however, that the entire relationship between parents and child is, of course, always a complex interplay, where both several senses and, of course, contextual conditions play an essential role."