Scientists are just now realizing how experiences after birth, rather than something innate, determine the actual wiring of the human brain. "Only 15 years ago," reports the Families and Work Institute in the just-re-leased study "Rethinking the Brain," "neuroscientists assumed that by the time babies are born, the structure of their brains [had been] genetically determined." But by last year researchers knew that was wrong. Instead, early-childhood experiences exert a dramatic and precise impact, physically determining how the intricate neural circuits of the brain are wired (NEWSWEEK, Feb. 19, 1996). Since then they have been learning how those experiences shape the brain's circuits.
At birth, the brain's 100 billion or so neurons form more than 50 trillion connections (synapses). The genes the baby carries-from the egg and sperm that made him-have already determined his brain's basic wiring. They have formed the connections in the brain stem that will make the heart beat and the lungs respire. But that's all. Of a human's 80,000 different genes, fully half are believed to be involved in forming and running the central nervous system. Yet even that doesn't come close to what the brain needs. In the first months of life, the number of synapses will increase 20-fold-to more than 1,000 trillion. There simply are not enough genes in the human species to specify so many connections.