Parental Hugs Calm Infants' and Parents' Nervous Systems

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Parent-infant hugs make both the infant and the parent feel calmer on a physiological level, according to a new study on parental hugging.

Notably, the researchers found that young children tend to have a more robust parasympathetic response to a fatherly or motherly hug than to a stranger's hug. These findings (Yoshida et al., 2020) were published on April 6 in the journal iScience.

"To the best of our knowledge, this study is the first quantitative and cross-sectional report investigating hugging behavior between parents and infants during their first year," the authors said.

According to the data, there appears to be a sweet spot for parent-infant hugging that lies between a loose "hold" and a "tight hug" that has the most calming effect on the nervous system.

This "Goldilocks Zone" of a hug that's "just right" is correlated with a boost in parasympathetic activity as indexed by increased interbeat interval (IBI) ratio between heartbeats.

The vagus nerve plays a fundamental role in parasympathetic activity and regulating heart rate via acetylcholine (ACh), which was originally referred to as "vagusstoff" (i.e., vagus substance). Vagus nerve stimulation releases vagusstoff, which slows heart rate and increases interbeat intervals (IBIs).

What does high or low IBI indicate? Increased IBI indicates that vagus-mediated parasympathetic activity is calming the autonomic nervous system. Conversely, lower IBI is correlated with increased fight-or-flight stress responses. IBI and R-R interval (RRI) are used interchangeably to measure the heart's electrical activity using an electrocardiogram.

As the authors explain, "Here, we examined the RRI increase ratio in infants and their parents during parent-infant hugs of the first-year infants. We observed an RRI increase, indicating parasympathetic activity in infants during the mother-infant hug but not when infants were being held by a female stranger or during a very tight hug from their mother."

As mentioned, the researchers also found that moms and dads showed increased parasympathetic activity during a parent-infant hug.

Why didn't the researchers use HRV to measure parasympathetic activity?

Heart rate variability (HRV) is the most widely used method for measuring parasympathetic activity. HRV parameters typically require recordings of autonomic regulation that are at least one minute in duration. Therefore, the researchers chose to use RRI to assess parasympathetic activity because most parent-infant hugs with children under 1 don't usually last for 60 seconds or longer.

RRI can determine changes in the autonomic nervous system in a matter of seconds. Faster readings were critical for this study because hugs in the "Goldilocks Zone" typically last for about 20 seconds before the first-year infant was inevitably "held" in a way that differed from a hug or a tight hug.

"Taken together, our results indicate that infants older than four months old may perceive hugs from their parents as a comfortable and relaxing experience, as opposed to 'holds' by their parents and hugs from strangers," the authors conclude. "The parent-infant hug may make both the infants and parents feel relieved and happy, which [fosters] parent-infant bonding."

"Your baby loves to be hugged," first author Sachine Yoshida said in a news release. "We hope that knowing how your baby feels while being hugged helps ease the physical and psychological workload of taking care of infants too young to speak."