Stock returns and negative skewness

CFA Level I Portfolio Management
#1

2014 CFAI L1 text Vol 4 P.296~297 and the concept of skewness introduced in QM

It is stated in the last paragraph :

… whereas Exhibit 9 demonstrate the negative skewness of stock returns by ploting a histogram of U.S. large company stock returns for 1926~2008, Stock returns are usually negatively skewed because there is a higher frequency of negative deviations from the mean, which also has the effect of overestimating standard deviation.

My questions :

  1. Is it that the mean in Exhibit 9 is zero ?

  2. Why there is a higher frequency of negative deviations from the mean (in Exhibit 9) ?

  3. Why it has the effect of overestimating standard deviation ?

#2

Any one can help me to understand the concept of skewness and standard deviation by examing the Exhibit 9 ? Thanks in advance !