How could he forget?

If distraction doesn't trip us up, absent-mindedness surely can. Among the most common reasons people forget important appointments or duties is blind devotion to routine. A person who takes the 10 Freeway west to the 405 north every morning en route to work is highly likely to take the same route on a morning when he or she has promised to pick up a relative at LAX -- which is south on the 405. The body is on automatic pilot, being cued by its environment to repeat familiar actions, while the brain dreams vacantly of coffee.

"If there's no cue available to remind you that the child is in the car, or you need to pick someone up, well, there's absolutely no limit to what people can forget," said Daniel Schacter, a Harvard University memory researcher who has studied legal cases involving heat-exposure death.

Mom is left waiting at the airport; dinner guests arrive at an empty house; a child is left sleeping in the back seat: These are all fundamentally the same kind of error, one in which preoccupation or absent-minded routine has tuned out working memory and muted any biological alarm system, he said.

Still, the people who leave children in cars often don't return for hours. Wouldn't they realistically remember their child five or 10 minutes later?

Without some hair-trigger neural system continually telling us to check on our children, after all, the species would hardly have survived, scientists say. This system is likely rooted in the most primitive brain regions, which can send emergency threat signals powerful enough to interrupt conscious thought: Where's the baby, check the baby. In time, as a child grows, parents may mute or amplify those signals, depending on whether they're dealing with a reckless thrill seeker or a bookish, careful child; whether they're at the playground or in the house, experts said. But some anxiety should continually flare to the surface.

Unless, experts say, the parent's head is miles away, in some engrossing project. Braver, the Washington University psychologist, tells the story of a former colleague, a father who took his child to the park in a stroller and soon fell deep into conversation with another parent. Lost in thought after the talk, the man walked to his car and drove off by himself, Braver said. Halfway home, the nickel finally dropped, and he sped back to the park. "The baby was fine," said Braver, "but he got so engrossed in the conversation he just forgot."