Conspiracy theorists offer a binder containing articles about the shooting of John F. Kennedy for tourists at Dealey Plaza. Corbis.

Amidst the grief, shock and confusion that followed the Boston marathon bombings last month, efforts to explain the senseless act inevitably led to theories of conspiracy and suspicious intrigue.

The bombers themselves may have been acting against what they saw as a conspiracy. Even the mother of the two brothers who planted the bomb declared the innocence of her sons, accusing the United States government of conspiring to kill her eldest.

What is it that motivates people to latch on to conspiracy theories, often in spite of evidence to the contrary?

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In some ways, it may be inherent in human nature to invest in certain beliefs at all costs, experts said, whether it’s about the health consequences of vaccines or the death of John F. Kennedy. Several basic psychological processes are at work.

For one thing, people feel better if they think they’re right, said Gary Marcus, a cognitive psychologist at New York University who wrote about conspiracy theorists for The New Yorker. In a phenomenon called “motivated reasoning,” it’s also common for people to both notice and seek out details that support their views and to reject evidence that might contradict what they want to believe.

Over time, people generally lose track of why they believe things in the first place, Marcus added, while continuing to strengthen their resolve. If there is strong emotional investment in an idea, such as the belief by a mother that she has raised a good son who would never bomb a public event, people will go to great extremes to resist evidence that their belief might be wrong.

Smokers are a classic example, Marcus said. When the Surgeon General released its first report about the dangers of cigarettes in the 1960s, non-smokers quickly agreed that smoking could cause lung-cancer. Smokers, on the other hand, were eager to point out that other things could kill you, too, that many smokers live long lives and that smoking is beneficial in other ways.

In his book “Kluge: The Haphazard Evolution of the Human Mind,” Marcus argued that the tendency for the human mind to retrieve information that confirms its thoughts is a design flaw -- making people feel good about themselves in the short term but reinforcing poor reasoning skills in the long term and ultimately leading to conflict.

“We’re pretty much all by default tending to notice things that fit with our theories,” Marcus said. “I think it’s a bug. We can’t search our memories for negatives. In a society where everyone gets a vote, when our brains work this way, it leads to gridlock.”

Boston Police look at blown out windows at the scene of the first explosion on Boylston Street near the finish line of the Boston Marathon.Boston Globe via Getty Images

Conspiracy theories seem to hold particular appeal for Americans, said Robert Goldberg, a historian at the University of Utah, Salt Lake City. One reason is that we’ve been raised on a diet of Hollywood movies that are full of sabotage, double agents and convoluted plots.

At the same time, he said, the U.S. government has long invoked conspiracy to disparage its enemies, from Communist supporters to militia groups. Even Hillary Clinton talked about the vast right-wing conspiracy that was out to get her husband.

Eighty percent of Americans believe that conspiracy was involved in JFK’s death, Goldberg said. And more than a third believe that the United States either had a hand in the 9/11 attacks or stood aside when they occurred.

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Only about 20 percent of Americans trust the government to do what’s right all or most of the time, according to a recent poll, he added. That decline of faith, combined with the appeal of a good story, sets people up to believe theories that involve secret plots.

Conspiracy theories also appeal when an event is especially horrible and when available explanations are plagued by ambiguity.

“Conspiracy theories give us a rationale, they give us reason to remove the randomness from what seem to be random, senseless acts,” Goldberg said. “They provide emotional sustenance and retaining power. They tell us what happened and why it happened and they point fingers at perpetrators. They give us targets that we can do something about.”

For some people, conspiracy theories are a fun way to speculate about what-ifs. But at their root, Goldberg said, suspicious hypotheses symbolize a lack of trust in an institution. Taken to an extreme, that same sentiment can motivate terrible acts like the Boston bombing.

“Lots of people for lots of different reasons become very fixated on different beliefs and sometimes they do so independent of the evidence,” Marcus said. “Most conspiracy theorists don’t commit crimes because it’s fun to talk about whether there was another shooter in the Kennedy assassination.”

“But some people cross the line and take a belief that’s important to them and act on it in the worst case in a violent way,” he added. “You probably don’t become a terrorist unless you have strong beliefs.”