Yes, but with other things... There is a say "If you say it enough times, you will believe it". The mind is an EXTREMELY powerful thing. I personally know people who had cancer and were given like 3weeks to live. They refused to believe, and thought otherwise... The cancer didnt go away, but it is like 80% less of what it was... and they are still living...

Same things applies to Soldiers and things alike.

That can also go into placebo...