Our objective was to propose a testable hypothesis arising from the recent 
finding of a low index-to-ring finger ratio (2D:4D ratio) in ALS. The 2D:4D 
ratio finding suggests that prenatal testosterone exposure may play a role in 
the development of the disease. Research from other fields is presented to 
suggest that healthy individuals with low 2D:4D ratio have enhanced sporting 
prowess, particularly with regard to activities requiring endurance and 
dependent upon slow-twitch muscles. Although studies are of varying quality, 
some epidemiological findings in ALS also suggest enhanced sporting prowess, as 
well as a higher risk of developing the disease among members of certain 
physically active professions. If the 2D:4D finding survives replication then 
this might explain the reported elevated risk of ALS among professional 
athletes, the military, and manual professions. Such a relationship might also 
explain why ALS patients were more likely to have been elite sportspeople in 
younger life. This hypothesis may serve as a starting point for debate and 
discussion over the nature of ALS risk factors, as well as generating a number 
of specific testable hypotheses that may yield insight into the genesis of the 
disease.