As soon as we are born, we begin figuring things out. We learn to ask for meals and fresh diapers, and we absorb an entire language between naps. Yet we recall little of those busy years. As it turns out, the same process that allows babies to learn rapidly might also disrupt the neuronal links that encode certain kinds of memories, according to a recent paper in Science.

Paul W. Frankland of the Hospital for Sick Children in Toronto and his colleagues suspected that the dramatic growth of neurons in the infant brain might interfere with the formation of memories. To test this idea, they ramped up neuron production in adult mice right after they learned something new. Unlike youngsters, adults grow neurons only slowly. But add exercise or drugs such as antidepressants, and neurons proliferate. When adult mice exercised after learning that a certain cage delivered a foot shock, the memory of the painful lesson diminished. Then the team turned to infant mice, whose brains naturally grow lots of neurons. These young mice rarely remember the foot shock for more than a day. When the researchers used a drug to slow the infants' neuron growth, the mice retained the memory for a week.

The findings suggest that new neurons disrupt the links that make a memory. In children up to about age three, the rapid-fire neuron growth that lets them learn about the world prevents memories of isolated events from sticking around. Infants and toddlers do form such memories—your 18-month-old may remember the dog she saw a few days ago—but unless the memory is reinforced, it will disappear as new neurons develop. After about age three, neuron growth slows, and the image of that dog may stick around for life. [For more on neuron growth in the brain, see “New Neurons for New Memories.”]