Although I haven't read their full paper, this seems like a total BS  to me. They could have as well said the hornier you are in the  beginning of the day, the more profitable your day is likely to be.
Basically, you collect some empirical data and run regression (or time series) and see if there exists relationship between  the length of the index finger and the profit they made trading. Then go to Biology books to look for reasons to support your claims, which is all cool and fine but to suggest that the length of your index finger matters as much as  your experience is, in my opinion, totally BS.