Looking for common denominators might lead us down the path towards new research:
Rh negative men and men with red hair report more mental health issues.
Rh negatives and people with red hair are more likely to be left-handed.
Both, Rh negatives and redheads are considered “more sensitive”.
High energy.
A lower threshold for pain.
Rh negatives are more likely to have red hair and redheads are more likely to be Rh negative.
Redheads and Rh negatives are more likely to have wavy hair.