In 2005 58-year-old UC Irvine neuroscientist James Fallon was looking at brain scans of both serial killers and his own family for two different studies he was conducting. The two types of scans just happened to be on his desk at the same time, but when he looked at both he noticed something shocking.

The brain of one family member looked a whole lot like the brains of the criminals. "I got to the bottom of the stack, and saw this scan that was obviously pathological," he told Smithsonian Magazine. Who was this secret psychopath in his own family?

When Fallon checked who the scan belonged to, he was in for the shock of his life. It was his own. The more he investigated, the more it became undeniable. Fallon was a brain expert who had missed the fact that he was a psychopath for his whole life. (Fallon tells his story in detail in his memoir The Psychopath Inside.)

How is such a thing possible? Thanks to Hollywood, we tend to think of psychopaths as knife-wielding criminals, but science shows some high-functioning psychopaths can actually be hard to spot. So hard, in fact, that some psychopaths don't know they meet the definition for the condition. Fallon, despite his profession, was one of this group.

Psychopaths can be surprisingly hard to spot.

Could you be a secret psychopath too? That's the subject of a recent fascinating Big Think Edge video with Fallon (subscription required). In it, Fallon explains that psychopathy is quite fuzzily defined by psychologists, and overlaps with other conditions like narcissism and antisocial personality disorder.

Confusing things further is the fact that many traits associated with psychopathy, like manipulativeness and ruthlessness in pursuit of goals, can be beneficial in some career contexts. Plus, they all exist on a spectrum -- you can exhibit them strongly or just a little.

Add that together and the result is some folks who meet the criteria for the condition function quite well in the world and never have cause to think they are on the psychopathic spectrum. But they meet the criteria for psychopathy all the same, and if they have some reason to look closely (such as a surprising-looking brain scan or a blog post), they can spot signs that they are a high-functioning psychopath, including:

1. You stay secretly angry for years.

"I asked the psychiatrist who has known me for many years what behaviors that are not so obvious that I do to people that would be psychopathic," Fallon explains in the video. "He asked me about revenge and my getting even."

Turns out Fallon's way of dealing with anger is classic psychopath. "Everybody gets mad, right? Somebody ticks you off, you get mad. You get mad for five seconds, 30 seconds, a minute -- everybody. It's a normal thing. And your serotonin kicks in after about five minutes and it cools you down."

Things work differently in the brains of psychopaths, even secret, high-functioning ones. They stay angry for months of even years, though no one would know it from the outside.

"When I get mad, I don't show it to anybody. I said I could be furious at you and you'd never know it. I show no anger whatsoever... I can sit on it for a year or two or three or five. But I'll get you. And I always do. And they don't know where it's coming from. They can't tie it to the event, and it comes out of nowhere," Fallon relates. If that sounds like you, you might secretly be on the psychopathic spectrum too.

2. You're good at externalizing blame in clever ways.

All Psychopaths don't feel guilt. They blame other people or circumstances when things go wrong, but high-functioning psychopaths like Fallon do this in particularly clever ways.

"If something happens, they will tend to blame somebody else, but it may not be in an obvious way. It may be a smart psychopath will externalize blame indirectly. So they'll take the blame and associate it with another occurrence," he explains.

So rather than simple finger-pointing, a high-functioning psychopath might craft a convincing story for why events or circumstances outside his control are to blame for any harm he may have caused.

3. You have ethics but not morals.

What does Fallon mean by the distinction between ethics and morals? "Psychopaths, if you watch them, they know the rules. They know the rules better than anybody probably. So they don't have to cheat at the game necessarily," he explains.

"But they don't have a sense of morality. They don't have scruples. And if you can see them in certain behaviors, you'll see they did something that was pointless and was just to give them pleasure, you know, something mean or something to use somebody where they had no sense of real morality, even though they followed the rules of the game," he continues.

Non-criminal psychopaths, in other words, know where the red lines are that can get them kicked out of a leadership role or other position of prestige and comfort. They are quite capable of controlling themselves enough not to cross those lines, as staying on the right side of the lines is in their own self-interest.

But while they might be capable of playing the game, they're going do it ruthlessly, without thinking about kindness or goodness at all, focusing exclusively on their own enjoyment and advancement.