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?
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."