He is blond and 3 years old, 33 pounds of compressed energy wrapped in OshKosh overalls. In an evaluation room at Yale's Child Study Center, he ignores Big Bird, pauses to watch the bubbles that a social worker blows through a wand, jumps up and down. But it is the two-way mirror that fascinates him, drawing him back to stare into the glass, to touch it, to lick it with his tongue. At 17 months, after several ear infections and a bout of the flu, the toddler's budding language skills began to deteriorate, his parents tell the evaluators. In the...