When one person feels a relationship is more than it is.
(from the movie Center Stage, when Jodi Sawyer believes that she is in a relationship with Cooper Neilson, when he just wanted to sleep with her and get away with it.)
Further study must be conducted before conclusive results can be reached in this new psychological disorder.
"Wow, she has stage one Jodi Sawyer syndrome."
Whenever anyone you've slept with gives you baked goods the next day and says something cheesy like, "This is for next time."
Whenever someone you've slept with on the fly refers to you as their significant other repeatedly. In front of your parents. And friends.