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.

When it comes to children, most parents are usually hyper-alert to threats, from uncovered electrical sockets to the neighbor’s dog. Yet in low-risk situations, when a child is, say, strapped safely into the back seat, they rely less on parental anxiety to carry them through than on working memory, the short list of thoughts and duties stored for easy access.

But it turns out that working memory is smaller and more fragile than is often assumed, said Nelson Cowan, a psychologist at the University of Missouri in Columbia who studies memory capacity.

Many of us can remember more than a half-dozen items at a time, for example, if they are related things, such as meetings or appointments scheduled throughout a workday, he said. But if the thoughts are unrelated, such as child care and work duties, then our heads usually hold only three to five things, Cowan has found. “For many people, child care falls into the ‘home’ category and work falls under ‘work,’ and there’s no overlap at all,” Cowan said. “The two are unconnected and competing for attention.”

In people of all ages, but especially those middle-aged and older, that competition can knock important thoughts or duties out of the ring. As anyone who has gone to the store without a written shopping list knows, the very act of remembering one thing on the list can immediately erase all the others, at least momentarily. “You think ‘bread’ and then you stroll down the bread aisle and now you’re distracted by all the different brands, and suddenly you’re at a loss -- you can’t remember any of the other things on the list.”

It doesn’t matter as much as we think that a child is far more important than a loaf of bread, psychologists say. In working memory, the two very different thoughts -- check on the baby, look for sourdough -- are still vying for limited space.

“The fact is that for most of us it is hard to hold onto even one thought,” said Michael Kane of the University of North Carolina at Greensboro, who studies distraction and working memory. In a series of experiments, Kane and Randall Engle of the Georgia Institute of Technology have shown that a flashing light on a computer screen is enough to confuse college students trying to accomplish a very simple task, such as turning their head away from the light.

The critical importance of remembering to pick up or care for a child can actually betray our better instincts to be vigilant, say memory researchers. “People think, ‘There’s no way I’m going to forget to pick up my own child,’ so they don’t worry about it enough,” Cowan said.

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.”

In a study done at Tohoku University in Sendai, Japan, and published in February, a psychologist had 19 men and women keep diaries over two weeks, recording lapses in memory and judgment as well as their moods at the time of the mistakes. Lapses were most common when the men and women were highly preoccupied with some project or problem. “Especially when people are doing something absolutely engrossing, something they love, they can get carried away for long periods of time,” said Braver.

The state of almost meditative concentration is similar to what the Hungarian-born psychologist Mihaly Csikszentmihalyi, a leading researcher in the field of creativity, calls “flow.” A widely recognized psychological state, flow is the total absorption that occurs when people are involved in an activity for its own sake, using their skills to solve a puzzle, complete a job or play a game, whether tennis or chess. It’s the equivalent of an athlete being “in the zone,” a jazz player losing himself in a melody or a computer science professor working out an especially challenging problem. During “flow” moments, one’s ego and sense of time are set aside.

It is, in short, a wonderful state of consciousness -- unless you’ve left a child in the car.

There’s no foolproof method for avoiding lapses altogether; we are all human, each with our own absent-minded routines, our engrossing hobbies or work. But psychologists say it does pay to be aware how flawed memory is, and how it usually goes wrong for ourselves, personally. Some people simply cannot hold more than a couple of thoughts in working memory at a time; others are prone to absent-minded routine; and still others are extremely tuned in -- usually.

“No matter how good your internal alarm, you can’t expect it to work every minute of every day for every person,” said Schacter. “The sad lesson we learn from these tragic deaths is that when it comes to memory, you can’t assume it won’t betray you.”