The Makedonian phalanx -- why such depth?

[Sean Manning]

I just came across an ancient source on the subject. The following is from Aelian's tactical manual written during Trajan's reign (tr. A.M. Devine). He describes a phalanx drawn up in close order (2 cubits per file) with pikes lowered and says: “[14.5] Therefore for each of those stationed in the front rank five sarissas project, presenting a forbidding aspect to the enemy, and thus each man is hedged about by five sarissas and pushes forward reinforced with the strength of five men, as can easily be seen. [14.6] And those behind the fifth rank, though not able to extend their sarissas, by pressing heavily with the weight of their bodies make the force stronger and deprive the front-rank-men of the opportunity to flee.” He was drawing on lost Hellenistic manuals.

It loooks like all of us have been forgetting the coercive factor! I suspect that's why very thin lines work in reenactment combat but not historically. And he seems to believe that the rear ranks helped by pushing somehow.