Say the stock is $100 and we are selling ATM puts and calls (European options). The call premium will be worth more than put premium by the amount equal to interest earned on strike price $100 prior to expiration at risk-free rate (put/call parity). This is based on pure arbitrage argument, no option pricing theory involved. i.e. buying one share of stock and a put is equivalent to buying a call and deposit cash of ($100 - interest). Intuitively I understand that since buying a call with cash deposit will earn an interest, a call must be worth more than a put. This is however from options buyer's point of view. For sellers who sell naked options, the margin requirement is the same for put and call. Does that mean selling calls gives you higher return on equity (assuming the put and call have equal chance to be ITM)? Or does this imply that the call is more likely to be in the money than the put at the expiration day? But why is that? (OK, Stock price goes up generally. But what about options on interest rate like TNX?) What am I missing?