The No. 1 sign you're a narcissist, according to a psychologist

Just because you post selfies on Instagram and write hyper-detailed Facebook statuses about your dinner doesn't necessarily mean you're a narcissist.

Common traits of narcissism include constantly feeling underappreciated, thinking everyone else is stupid, and feeling justified in acting mean to people.

But there's one key trait that distinguishes narcissists from everyone else: an absence of interest in other people and the inability to feel for them.

That's according to Joseph Burgo, Ph.D., a psychologist and the author of "The Narcissist You Know."

You'll know if this description fits you, Burgo said, if you lose interest in group conversations when they're no longer about you. Or if you feel completely indifferent when people talk to you about their emotions and issues they're struggling with.

Burgo estimates that at least 5% of the population demonstrates this quality and falls into the category of what he calls "extreme narcissists." In general though, he's noticed an increase in "everyday narcissism." For example, you might go to a party and talk to someone who tells you all about their own life, without asking you anything about yours.

The problem with acting this way, Burgo said, is that "being interested in other people is the basis of human relationships." It's virtually impossible to develop a deep connection to anyone if you fundamentally don't care about them.

Fortunately, Burgo said, "it's possible to try and develop an interest in other people."

In a way, it's an act of imagination - you have to put yourself in another person's shoes and envision exactly what he or she is thinking and feeling. That way, you'll encourage yourself to demonstrate empathy for others, even if it's hard for you to do.

Ultimately, addressing your narcissistic behaviors will be a worthwhile endeavor. As Burgo said, the ability to relate to others and see things from their perspective "kind of defines your life."