Why are games more likely to last 60, 80 and 120 moves? I've downloaded and parsed the king-base chess database (about 1.5 million games) and am looking at distribution of moves for games. When plotting the distribution for different outcomes, I see spikes at 80, 100 and 120 moves (and smaller spikes for draws at 20, 40 and 60 moves).



Why are these spikes occurring? Are people more likely to resign at these round numbers? Or are there rules in professional games that mean games are more likely to end after multiples of 20 moves?
This is due to time control:


  the World Chess Federation FIDE sets a single classical time control for all major FIDE events, according to the handbook: 90 minutes for the first 40 moves, followed by 30 minutes for the rest of the game, with an addition of 30 seconds per move


Other time controls are used, but are usually based on multiples of 20 moves.

Mikhail Botvinnik, a former world champion, once said:


  The amount of errors made at time control are legion


So gross errors accumulate around the 20th, 40th, 60th etc. move, and so would resignations, loss on time and draw offers.