It's a situation parents know all too well: Even if a baby seems completely calm, there’s a good chance they’ll start fussing the moment you sink into the nearest armchair. If you’ve ever spent a night pacing endlessly with an infant in your arms and only one thought in your head—“Why won’t my child let me sit down?”—we have some answers for you.

As Fatherly explains, the rather exasperating phenomenon could have evolutionary origins. Imagine that you’re tearing through the jungle with a bloodthirsty predator in hot pursuit; your odds of survival are higher if the child clutched to your chest isn’t slowing you down by squirming and wailing. Over millions of years, our species might have evolved to automatically stay still and quiet when our parents are on the move, and return to our former state of restlessness as soon as that movement stops.

In a study published in a 2013 issue of the journal Current Biology, researchers tracked the heart rates of 12 infants in three different situations: while their mothers were carrying them; while their mothers were sitting with them; and while they were lying in a crib. In addition to visibly calming down when carried, the babies’ heart rates were noticeably lower.

“These data suggest that infants were more relaxed during carrying than during holding, not only behaviorally but also physiologically,” the researchers wrote. You can see some of those behavioral and physiological effects in the video below; the graph charts the time elapsed between beats, so high spikes correspond to low heart rates.

An innate physiological response to being carried isn’t specific to human babies. Other mammals, including cats, rats, and lions, curl their hind legs up and grow still whenever their mothers pick them up. In the same study, the researchers observed that mouse pups “maintained an immobile and compact posture” when their mothers grasped them by the napes of their necks.

“This study is the first to establish the striking similarities of the carrying-induced calm state between human infants and mouse pups as an orchestration of reduced mobility, distress vocalizations, and heart rate,” the study said.

Of course, we can’t definitively conclude from one small study that human babies are, in fact, unwittingly trying to help us survive by quieting down whenever we walk around. But the hypothesis alone might still allay any worries that your baby has a mean-spirited sense of humor.

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