This strengthens the argument by suggesting difference in athletic ability (which is not an environmental factor) might be the predominant cause of differences in sports participation among members of the same family.

Comparisons of enthusiasm between teens and parents don’t have any clear impact. Nothing suggests that this difference is due to environmental factors.

This tells us about how adults’ enthusiasm relates to the same adults’ enthusiasm when they were younger. But this doesn’t reveal anything about what might have caused their enthusiasm when they were younger.

Different societies/decades involve different environments (ex. think about cultures in different countries and times). If environment had little impact, we’d expect sports participation not to vary widely. But it does, suggesting environment has more than little impact.

If anything, this might strengthen the argument by providing additional evidence to question the effectiveness of programs designed to get teens to participate in sports. If only a few are highly successful, maybe it’s just coincidence. The program might not be the cause.