To all the math folk out there...

Question posed today in class (and no, it's not supposed to be a math class...yet)

You are hosting a party, with your significant other. You invite 20 couples (yes, it must be couples). Sometime during the party, you go around and ask each person how many people he/she has shaken hands with. The person cannot have shaken his/her SO's hand, or his/her own hand. Each person gives you a different number.

How many people did your SO shake hands with?

Why do you think your answer is right? I did end up with the right answer, however, my logic was flawed. And very very wrong.