Babies are born with just about all the neurons they'll ever have -- about 100 billion of them. Though only one quarter the size, a baby's brain looks exactly like ours.

Over the next three years, that brain triples in size, establishing more than 1,000 trillion intricate and complicated connections between neurons. Anyone who has cared for a child has witnessed this explosive growth, from that ungainly infant to a running, shouting toddler with a sense of self and opinions to share.

This magical and sometimes stressful window of rapid growth, when the brain's malleability is at its peak, can set the stage for future learning and healthy development throughout life, according to neuroscientists and child development experts. "Most aspects of brain development after birth depend on experience occurring during this sensitive period," he says. If those experiences don't happen, "then development can go awry. The longer the brain goes without those critical experiences, the harder it is to recover from that."

Our understanding of this sensitive period has helped explain why Romanian orphans raised in institutions with little normal stimulation, children Nelson has researched for more than a decade, can suffer from profound deficits in learning and behavior. It's also why many can recover if they're placed in good foster care before too many years have passed.

It's also why there's so much attention, funding, resources and interventions for low-income children, who are the most likely to suffer neglect, aimed at this time period. Children can learn and grow throughout their lives, but the payoff for heading off trouble early is theoretically the highest, the experts say.