An academic paper looks at the statistics of penalty shoot-outs, and concludes that the psychological pressure plays a pretty prominent role. Before you go "D'oh!", read this summary which discusses some of the implications of this finding:
For the first kick, when the pressure is relatively low, an average of 87% of kicks were successful. But the rates of success then start to drop, down to 73% for a fourth shot, when the pressure is often higher. The success rates could be partly influenced by coaches picking their best players to shoot at certain times. But that can't account for all the difference, Jordet says.
The results are even more dramatic for the highest-pressure situations. When missing a kick means defeat for the entire team, the success rate plummets to 52%. But when a successful kick guarantees a win, 93% of attempts go in.