What is the average penis size?

There have been several studies regarding the average size of the human penis. The majority of such studies could be unreliable due to self-selection bias: men with a smaller than average penis might be less likely to allow themselves to be measured, while men with a larger than average penis might be more likely to allow themselves to be measured. It is unknown if this conjectured self-selection bias has been confirmed by any studies, or if any studies have attempted to account for potential self-selection bias regarding penis size.