Quote from Algorithm:
If I understand the original question as; why do long puts and long calls have a price differential when the underlying equity is ATM?
First, this is a good observation and has been debated for a long time.
Second, the best answer I have found has to do with intrest rates and the underlying value of time on money. It goes something like this, when you buy the call you are deferring the purchase of the underlying stock and some of the inherit risk in owning the stock outright (mainly control the shares without the higher cash outlay). Higher interest rates widen the difference between put and call prices, only at a 0% real interest rate would an exact price correlation be 1 to 1 in regards to the puts and the calls. The cost of carry is really the main culprit for the price differential betweent he long calls and the long puts on the underlying equity when it's ATM.
Exactly, the interest component that is included in the call price is the reason.