This is unscientific, but almost 100% accurate. A girl who uses a guy’s first name (David if he goes by Dave, Matthew if he goes by Matt, etc.) is a red flag that she’s trying to make him into someone he’s not and more than likely going to turn into “that one crazy girl I dated.” Do yourself a favor and RUN AWAY from these girls, guy.
And girls, I have also found guys who go by a full name when there’s an easy nickname tend to take themselves too seriously. Or if it’s coming from parents who never called him a nickname, THEY’RE probably too serious.
Like I said, unscientific. And I’ve found the first case to be almost 100% true. The guy/nickname one is less accurate, but still seems to be a good litmus test.