That was one of the initial reasons I raised the questions earlier in this thread (review it). In addition to delta> 0.5, saying ATMF delta is equal to 0.5 is equivalent to saying price of option is zero. A price of an ATMF call option is positive (nonnegative) is equivalent to saying its delta is positive (nonnegative).
I am not familiar with Derman and his/her work. How did he/she prove that delta is >0.5? It would be useful to reproduce his/er arguments/proofs here. Others gave other good insights (review thread).
I am not familiar with Derman and his/her work. How did he/she prove that delta is >0.5? It would be useful to reproduce his/er arguments/proofs here. Others gave other good insights (review thread).
