It's not always easy to follow your heart. But for human babies and their mothers, following each other's hearts may be as simple as sharing a smile. A new study shows that 3-month-old infants and their mothers can synchronize their heartbeats  to mere milliseconds. Researchers sat 40 pairs of mothers and infants face-to-face, equipped with sticky skin electrodes on either side of their hearts. Beat for beat, mother-and-child hearts thumped together almost instantly as they shared loving looks or contented coos. This cardiac coupling worked only for moms with their own babies, and only when the duos synchronized smiles and other cheerful social behaviors, researchers report in this month's issue of Infant Behavior and Development. When humans mirror each other's facial expressions, they may switch on specific areas in the brain that tell the heart when to thump, the researchers suspect. Melding with mom lasts longer than just a few beats, however. Babies who don't tune in with their mothers are less empathetic as teenagers , according to previous work from the same lab. Premature infants or those whose mothers have postpartum depression may be most at risk for losing this social skill because they miss out on early opportunities to interact with mom.
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