If an otherwise smart, older person in your life seems to be disregarding advice (especially this column's) on spending money, trust to your instincts and intervene.

Otherwise, you'll find yourself smacking your forehead.

adds a growing body of evidence that a particular part of our brain helps us detect deception. When it's compromised, all bets are off. Or should be.

And it becomes increasingly less able to doubt in old age, researchers say. This "doubt deficit," as researchers call it, leaves older people vulnerable to deception and fraud.

"You can do everything you've always done," explains Don Blandin, chief executive of the

in Washington, D.C. "But when it comes to financial investments, you need help."

First, let's review a bit about what we've learned about the brain.

Increasingly, scientists think our brain initially believes everything it takes in. That piece of pizza in

? Your brain probably reacts, thinking, "I like pizza. I want pizza."

More on money and aging

Read Brent's 2009 column

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Read the study that column highlighted:

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Watch the Investor Protection Trust's

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Visit the Oregon

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Doubt and skepticism involve a more complicated and deliberate thought process. Tempering that initial thought with "I just bought pizza last night" or "I need to watch my weight" takes more effort and a bit more time.

This two-pronged thought process is the focus of a best-selling book by Nobel Prize-winning economist

called "Thinking, Fast and Slow."

Doubt and skepticism occur in the prefrontal cortex, smack behind your forehead. It's the part of the brain that

until you turned 25, what made you finally heed advice about diverting money to your 401(k) at the expense of new wheels and threads.

Unfortunately, our prefrontal cortex begins to deteriorate again at age 60 or older, though its onset and rate of decline varies widely.

Damage to that part of our frontal lobe reduces our ability to detect misleading advertising, new research says.

In a study released in July in

, researchers and a marketing professor at the University of Iowa showed eight print ads to 39 brain-damaged patients and 10 healthy patients.

The ads mimicked real ones that had been dubbed misleading by the Federal Trade Commission. For example, the headline on the study's mock ad for "Legacy Luggage" claimed: "Legacy brings you the finest American Quality luggage." The luggage, however, was made in Mexico.

Eighteen of the brain-damaged patients had damage to their prefrontal cortex. Those 18 were more apt to believe the misleading ads, relative to healthy patients or those with damage to other areas of the brain, researchers found.

They also seemed more intent on buying the products hawked in the ads after seeing them.

In fact, they were more likely to do so even when given information that contradicted the ads' claims.

A "NatureCure" ad, for example, claimed to provide headache relief without side effects. It also had a disclaimer at the end saying it could cause nausea in some consumers.

"Indeed, this deficit may explain why highly intelligent prefrontal cortex patients fall victim to obvious fraud," the authors said.

You probably know someone like this. Such people can be "obstinate and bullheaded" in the face of new information, once they've made up their minds about something, the authors said.

"Warnings from friends and family often go unheeded," the authors wrote. "Susceptibility can result in bankruptcy if they continue to make their own financial decisions."

Unfortunately, friends and family might also be the perpetrators.

by the Investor Protection Trust and Investor Protection Institute asked more than 700 adult caregivers, financial professionals and other experts about elder investment fraud.

About 80 percent cited theft or diversion of money by family members among the most common types of deceptive practice targeting older Americans. Nearly 50 percent cited theft by caregivers and scams perpetrated by strangers.

What can you do about this? Here, science is of less help.

Erik Asp, one of the study's authors, said researchers still don't understand enough about how the brain works to know how best to improve it.

Researchers studying dementia think that the more people use their brains, the better they'll stave off such diseases, he said.

"Perhaps one thing older individuals might do is read books rather than watch TV every night," said Asp, a neuroscientist now at the University of Chicago.

has shown that patients with damaged prefrontal cortices are less likely to resist authoritarian persuasion and more likely to hew to

, associate professor of neurology and psychiatry at Oregon Health & Science University, studies Alzheimer's disease and other forms of dementia. She says you can stave off cognitive decline by eating nutritious foods and getting plenty of exercise.

You can also curb the risk of vascular disease, which harms the brain. That means avoiding smoking, controlling blood pressure and, if diabetic, keeping blood sugar in check, Howieson said.

But don't wait for telltale signs of vulnerability: unusual judgments, isolation, hoarding. One-third of Americans 80 or older suffer from dementia, Howieson said. As the baby boom generation ages, the prevalence of dementia could triple by 2050 unless interventions are found, researchers say.

Talk caringly to older parents and friends now about how to manage their affairs. Do it while their frontal lobes are intact, and we all still have a chance.

"It's better to talk to parents while parents can still have input," said Blandin of the Investor Protection Trust. "Especially the baby boomers. We're a real stubborn bunch."

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has never had a brain scan, but welcomes questions about his column or blog. Reach him at 503-221-8359. Follow It's Only Money on

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