When I was in med school, there was a classic study that asked the queston:
"If male, are you circumcised?" 85% said yes.
When they followed up, with actual examination, it turned out only 49% were correct. Think about that for a minute....
About half of all voluntary male respondents to a written survey
did not know what their penis looked like.
Yeah - so even surveys about what your genitalia is like is 50/50 for accuracy. So since then, if someone shouts survey studies at me, I just laugh. And check my penis.