Any infant left on a doorstep can safely bet that whoever finds her will be compelled to protect her. Nature cleverly designed babies to be super cute: They would be too much of a drag to care for otherwise.

As zoologist Konrad Lorenz first pointed out, we adults seem to respond favorably to the typical infant facial structure—disproportionately large heads and eyes, small noses, and chubby cheeks (modeled here by my delightful neighbor.)

A new study out of the University of Oxford used brain imaging to reveal a glimpse of the biological basis of our collective cooing over babies: Morten Kringelbach and Alan Stein showed that a brain region associated with our emotional responses to rewarding stimuli lights up within a mere seventh of a second in response to pictures of babies, but not in response to shots of adults.

Another recent brain imaging study published in Biological Psychiatry shows how mothers' brains light up differently in response to images of their own babies crying and laughing than to images of other peoples' babies. While we're all predisposed to feel warm and loving toward infants, each one has a particularly vigilant advocate in his or her mother.

These discoveries could eventually help researchers treat mothers who have trouble responding to their infants' cues, including those who suffer from postpartum depression, which affects about 13 percent of new moms.

As we've covered here, grown-ups who never outgrow that babyish look keep eliciting protective responses from others. Leslie Zebrowitz, professor of psychology at Brandeis University and expert on snap judgments, has shown that baby-faced men are seen as innocent and weak. Some, such as Eminem, compensate by building up tough-guy images.

A baby who responds with a smile or laugh is even more captivating than one who simply looks cute. Which is why I can't resist wrapping up with this tape of a hysterical Swedish tyke:

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