Criminal Minds, Season 4

Viper:
You see, eye contact is a very powerful gauge. That's why you tend to look away from someone you're attracted to, because you know, instinctively, what a dead giveaway it is. But your brain goes there anyway; images, fantasies.

Jordan Todd:
Don't flatter yourself.

Viper:
The eyes don't lie. They dilate. It's a chemical response we can't control.

Emily Prentiss:
Okay, 15 seconds.

Jordan Todd:
What do you see?

Emily Prentiss:
No, no change. No dilation.

Jordan Todd:
I guess, chemically, we aren't a match.

Viper:
Only because you have someone else on the mind. Once that happens, the attraction center in the brain shuts down.