请教各位大神一个关于volatility skew 分布的问题

kentcfa

2016年练习题里面有这样一个题

The observed volatility skew for a particular equity index slopes downward to the right. Compared to the lognormal distribution, the distribution of option prices on this index implied by BSM model have:

答案的注解是这样的：

A downward sloping volatility skew indicateds that out of money puts are more expensive than predicted by BSM model and out of the money calls are cheaper than expected predicted by BMS model. the implied distribution has fat left tails and thin right tails.

请问一下为啥是左肥右瘦而不是右肥左瘦呢？ 如果没有skew,只看smile的话，不是应该左右都肥吗？我的理解是波动性高的应该有肥尾巴呀。

谢谢指导一下。