What does research reveal on the right mix of POSITIVITY and NEGATIVITY in mindset?

In his book To Sell is Human , Dan Pink mentions that for effective selling , being overly POSITIVE and PUMPING oneself up is not the most appropriate solution.

A right amount of negativity and interrogative self talk is necessary to throw caution to the winds for effective sales performance

The most appropriate ratio of POSITIVITY to NEGATIVITY has been found at 3:1 when the sales performance tips.

This teaches us why cautious optimism and productive paranoia are key to a salesperson’s repertoire.