From the last paragraph of TFA:
The findings do not directly link playing video games to reckless horse riding. They only show an association. Researchers say the impact of playing games like "Red Dead Redemption: Grand Theft Pony" is minimal.
Bingo. Horse riding games could cause reckless horse riding in real life. Or people who ride recklessly enjoy horse riding games. Reckless pony racing could also be associated with both games and horse riding, but that wasn't the focus of the study.
The IQ of the group is the lowest IQ of a member of the group divided by the number of people in the group.