Babies don’t make it easy on moms. They cry, whine and hold on for dear life if their mother tries to even leave the room. But a new study suggests an easy solution to help loosen one of the strongest forces in nature, the bond between mother and infant: a worn piece of clothing.
According to research published today in Science Advances, chemical signals in a mother’s unique scent may help infants bond with strangers, even when the mom is absent.
In the study, babies were exposed to their mother’s body odor via a worn t-shirt while interacting with a stranger. The infants that had their mother’s scent present, versus those exposed to a clean t-shirt, were able to bond with the new adult more easily, a finding that may help parents share child-rearing duties. “What's so exciting about it is that we can delegate parenting to those we live with,” says Ruth Feldman, a social neuroscientist at Reisman University in Israel, and co-author of the study. By simply having the scent of the mother present, the results suggest, another adult can fulfill a similar role in a child’s social and emotional development.
Body odor helps us connect with and understand others, explains Feldman. “From an evolutionary perspective, what is the secret to Homo sapien’s survival, thriving and conquest of the world: our capacity to communicate,” she says. But our sense of smell, which was one of the first senses humans evolved, has often been overlooked in the study of infant-parent communication compared to vision and hearing.
One reason social smells are less studied is logistical: scents are complex and hard to manipulate in a lab. Body odor isn’t a single scent, but rather a cocktail of chemical signals. Another reason smells are less studied is because humans are unconsciously sampling the smell of our surroundings all the time, says Eva Mishor, a neuroscientist at Weizmann Institute of Science in Israel, who wasn’t involved in the work. Smells “affect our mental state, our brain activity, our hormonal state and our social interactions,” often without us noticing. “But if we look at the main decisions that we make in life, if it's what to eat, who should we mate with—sense of smell is a very important part of this,” she says.
Feldman and her colleagues knew from previous research with rodents that odors can be critical for an offspring's ability to recognize and bond with its mother, but few studies have examined the phenomenon in humans. The team was particularly interested in how a mother’s scent impacted the behavior and brain activity of babies ages 5 to 10 months. That age window is important, says Feldman, because a baby’s “social brain”—areas of the brain that are responsible for emotional regulation and social connection—develops rapidly.
Feldman was also curious how the presence or absence of a mother’s scent would impact the brain-to-brain synchrony between an infant and adult. During a social situation, human brain activity starts to correlate with the brain activity of those we are interacting with, in a process called “neural synchrony.” A conversation or eye contact is enough to get two individuals’ neurons firing in similar areas of their brain. This type of mirroring between parents’ and infant’s brain has been linked to the child’s emotional development, which made Feldman wonder how odor might impact the level of neural synchrony between a mom and baby.
In their study of 62 women, Feldman and her colleagues gave each mom a cotton t-shirt to sleep in for two consecutive nights. During the day, the mothers stored their shirt in a glass jar in the freezer. Then, each mom brought their baby and their worn t-shirt into the lab, and researchers placed EEG electrodes on the participants’ heads to measure their brain waves as they interacted under various conditions.
The first scenario was designed to give scientists a baseline understanding of the brain-to-brain synchrony of moms and their babies and didn’t involve the t-shirts at all. Each mom and their child was first seated facing back-to-back, and then face-to-face, with the moms in chairs and the babies in highchairs. As the researchers anticipated, they found that neural synchrony was higher between the mom and infant during face-to-face interactions. Then, they repeated the setup with the same set of infants, but this time introduced a female stranger who lived in the area and had a child of a similar age. As the infants interacted with the stranger, Feldman and her team placed either a clean or worn t-shirt on the highchair tray or near the baby’s face to see if the nearby smell of mom changed the infant’s behavior.
They found that infants presented with a clean t-shirt showed significantly lower brain-to-brain synchrony when interacting with the stranger, compared with when they interacted with a clean shirt and their mother. But when the babies were exposed to the mom-scented t-shirt, they showed the same degree of neural synchrony in both the mother and stranger scenario. “When infants interacted with the ‘stranger mother’ in the presence of the mother's body odor, the brain-to-brain synchrony leveled up....as if the infant trusted the other woman,” says Feldman. In addition to heightened neural synchrony, the results showed that infants exposed to their mother's scent during an interaction were more relaxed, smiled more and made more eye contact with the stranger.
Finding that a mother’s scent plays an important role in a child’s comfort is “a nice scientific confirmation of what many parents probably already do,” says Sarah Jessen, a neuroscientist at the University of Lübeck in Germany, who wasn’t involved in the study. If a mom leaves for work, for example, she may leave a blanket or piece of clothing with her scent to comfort her child. Jessen says this research could potentially be applied in medical settings too, like when a mom is separated from their child in a hospital. Next, Feldman and her colleagues are repeating the experiment with fathers.
The field of odor research is growing so rapidly, says Mishor, that this study is “just scratching the surface.” She is curious to see if they will be able to pinpoint a specific molecule or chemical in the mother’s scent that is driving processes. For now, she says parents can take comfort in the fact their baby is smelling and remembering their unique scent, and that scent could help the infant bond with others. “We sniff our babies all the time, and they sniff us,” says Mishor. “It’s meaningful for us, and for them.”