How could he forget?

Being a parent means being always on edge, always vigilant. That's why so many were dumbfounded to hear reports recently that a UC Irvine professor had driven to work and forgotten to take his 10-month-old son out of the car. The boy later died of heat exposure, as have at least three other young children left unattended in cars in the West this summer.

How does a decent, responsible person neglect the most important thing in his or her life? You can count the ways, say experts on memory and human error.

"Any parent who looks at these cases and thinks, 'I'm confident that could never happen to me,' should stop and think again," said Todd Braver, a neuroscientist at Washington University in St. Louis who studies memory.

About 30 children die each year of heat exposure in the United States after being left unattended in hot cars, experts estimate. In just the past two months, two boys, ages 3 and 5, died after being left for five hours in an SUV near Lancaster; and a 7-month-old left in a hot van in Las Vegas died after his dad forgot to drop him off at the baby sitter's. Many more are left by accident in stores, at home or, in one 1987 case, in a car seat placed on top of the car; the mother drove two blocks before her 2-month-old fell off, injuring his collarbone. While some of these mishaps are due to neglect or resentment, experts say that most are tragic blunders by otherwise responsible parents.

"These kinds of errors are made all the time, and for most of us we're just fortunate the consequences aren't so tragic," Braver said.

For years, psychologists and other neuroscientists have searched for common denominators in cases of serious human error. They have done laboratory experiments on distraction. They have had people keep detailed diaries of their memory lapses. And they have tried to recreate the precise circumstances and thinking (or lack thereof) at the time.

The evidence suggests that people make awful, life-changing mistakes for the same reasons they forget to pick up the milk: because they misjudge the reliability of their own memory; because they're highly distractible; and because they're generally unaware of how easily and completely they can become engrossed in pet projects or problems, absent to everything else.