Let's say we live in a world where it is not clear who is adopted and who is not. Most believe a Billion or less are non-adopted, and that they themselves are non-adopted. A few say everyone is non-adopted, some say being non-adopted isn't even possible.

If you believe just 13 million* people worldwide are non-adopted, you need good evidence to believe you happen to be one of them.

Believing you're not adopted based on little evidence makes perfect sense if you're in a world where the vast majority of people are not adopted (or know they are)

*13 million=LDS membership (wikipedia)