Narrator: This is Science Today. Most parents have no doubt heard about, and perhaps feared, that time in a child's life coined "the terrible twos". Alison Gopnik, a psychology professor at the University of California, Berkeley says as exasperating as toddlers can be during this rebellious period, it's a time that's crucial to their cognitive development.

Gopnik: What we've done is a bunch of very careful experiments to show that between the time babies are born and the time they're about four, they're changing their ideas about how other people work in very regular and systematic ways.

Narrator: This is done during everyday interactions with people.

Gopnik: So, you can think of the terrible twos as being a kind of experiment that comes when you're eighteen months old and you suddenly get this new, startling hypothesis about other people which is, "my God, maybe sometimes they don't want the same thing that I do! Let me check this out and test out ideas - especially about how other people work.

Narrator: For Science Today, I'm Larissa Branin.