User Registration/sign up displays default name, how can i change the default setting

When a USER signs up or I add one manually, the default public display is the REAL NAME.
Users prefer to display the USERNAME. How can we change the default setting. Yes they can re-login and change it but not everyone knows it so the first default should be the USERNAME.
The membership displays the actual names of the users by default.
How do I change it to default Username?
Most people don't want their real name displayed publicly.

thanks

Aurora