And let's set aside violent people and liars - how can you just tell if someone is more likely to be a jerk? Give their clothing a look. A neat and formal appearance just says they're conscientious .

But are their duds expensive? Is a woman displaying cleavage? Is a guy showing off his muscles? Hello, narcissism . Here's Sam Gosling again:

(To learn how to deal with a narcissist, click here .)