When, in a situation such as a double date or casual hangout involving two girls, it can clearly be noticed that one girl is prettier/smarter/funnier/more superior than the other, much like how Marsha was always better than Jan in The Brady Bunch. Therefore, the poor girl who is inferior to the other seems to experience the Jan Brady Effect.
Troy: "Wow, Gavin. You sure do have a 10 right there!."
Gavin: "Yeah, she's quite beautiful. I'm very lucky. I just feel kind of bad for your girl. Candance seems to just be all around better than her."
Troy: "Yeah. We clearly have the Jan Brady Effect going on here."