When your baby is crying, it feels nearly instinctual to stand up to rock, sway and soothe them. That's because standing up to calm babies is instinctual—driven by centuries of positive feedback from calmed babies, researchers have found.

"Infants under 6 months of age carried by a walking mother immediately stopped voluntary movement and crying and exhibited a rapid heart rate decrease, compared with holding by a sitting mother," say authors of a 2013 study published in Current Biology.

Even more striking: This coordinated set of actions—the mother standing and the baby calming—is observed in other mammal species, too. Using pharmacologic and genetic interventions with mice, the authors say, "We identified strikingly similar responses in mouse pups as defined by immobility and diminished ultrasonic vocalizations and heart rate."'

This is even more proof that, as daunting as motherhood is, our hearts and bodies already know so much of what to do. From the way we talk to babies to our "spidey senses" when something is amiss, these things we do without second thought come from deep within us.

In the case of standing with babies, the researchers concluded the "infant calming response to maternal carrying is a coordinated set of central, motor, and cardiac regulations and is a conserved component of mammalian mother-infant interactions."

So why do babies stop crying when we stand up? Because being carried induces physiological responses that calm and soothe babies. Being carried by mom induces "central, motor, and cardiac regulations" that have a calming response stronger than rocking or other kinds of movement. Our babies evolved to want to be carried (which is good because sometimes mama has to do things that can't be done from a rocking chair).

In other words, it's an example of us working perfectly in sync with our babies. So, on a day where everything feels really, really hard, it can help to remember you know what you're doing. Even, or especially, when you don't pause to think about it.

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