The most effective realism isn’t the opposite of or a lack of positive thinking, but rather a lack of naivety per se.
The distinction between the two is subtle, but it makes all the difference.
The most effective realism would actually take place in conjunction with positive thinking. A study has even shown, I read once (I couldn’t find a reference), that positive thinking “paradoxically” correlates with success more so than realism. (I imagine they meant realism as we normally think of it, as being aware of the facts and principles involved but also lacking a certain element of positive expectations for fear of being unrealistic and hence being disappointed.)