What gives home sides an advantage over visiting teams?

Readers answer other readers’ questions on subjects ranging from trivial flights of fancy to profound scientific conceptsStatistics show that there is such a thing as home advantage in team sports. Presuming that the (away) athletes’ skill levels don’t alter, their effort remains the same, and are professional enough not to be traumatised by the home fans’ noise, what could be the reason for this?
Dave Sidney, London• Post your answers – and new questions &ndas