This should not be surprising since an equity investment is a call option on the residual value of the firm. Increases in valuations are associated with growth and volatility. You have to be positive about growth prospects to hold a portfolio of call options.
Why is this bias discussion important? The optimist will have the bias of looking for positive information or filter information in an attempt to look for the positive. The pessimist will always use a negative filter and look for the bad news in the data. A model, however, will not have either bias, so systematic investing can eliminate this most fundamental bias.