Dr. Siegbert Tarrasch was a German medical doctor and one of the finest chessplayers of the early 20th century. In his book on the St. Petersburg 1914 International Chess Tournament, he mixes chess and medical commentary as follows. In the game Gunsberg-Blackburne, commenting on white’s weak 17th move [if you click on the link you can see the game], he says:
Why does Gunsberg, at an age when Andersson and Steinitz were still enormously strong, show scarcely a trace of his former strength? And why are the beaux gestes of Blackburne, a 73 year old man – one cannot say an old man – still so acceptable? Could it be the power of alcohol, which Blackburne consumed in considerable quantities all his life and which proved to be a medium of preservation for him, while Gunsberg is an outspoken teetotaler? Blackburne’s case is a phenomenon that the temperance union must explain, for it is appropriate for reducing their efforts directly ad absurdum.
This explains everything. It is my very low rate of alcohol consumption (I only consume a bit more than Mitt Romney) that has caused my chessplaying ability to deteriorate over the last 15 years. I fear to think of what Dr. Tarrasch would prescribe for me after looking at my games.
That, and perhaps, the lack of well-trained competition.