First, a few studies that attest to the benefits of optimism:

Prati & Pietrantoni (2009): This meta-analysis of 103 studies concluded that optimism contributes to posttraumatic growth, probably through optimism’s role in  promoting effective coping strategies. The authors noted three main findings of optimism research: “First, optimistic persons might be more inclined than pessimists to derive a sense of benefit from adversity... Second, it is known that optimism is not related to rigid patterns of coping strategies, irrespective of stressor type. In fact, optimism is related to flexible use of adaptive coping strategies with regard to the controllability of stressors...Third, optimism is considered a predictor of perceived capability to manage the demands of a potentially traumatic event.”

Micahel, Hou, & Fan (2011): The authors collected longitudinal data from 120 spa employees. Controlling for the effects of job tenure and personality traits, they found that employees with high confidence in their creative abilities demonstrated a high level of innovative behavior at work and that of these workers, those “with greater optimism exhibit greater innovative behavior at work”

Nicholls, Polman, et al. (2008): Mental toughness was defined as a combination of perceived control, commitment, positive attitude towards challenge, and confidence. The authors note that previous research had shown that more optimistic individuals exhibit increased effort to achieve goals and less optimistic individuals are more likely to withdraw or disengage attempts at achieving a goal. In their own study of 677 athletes, they found that higher levels of mental toughness were associated with problem-solving coping strategies, which in turn were associated with higher optimism.

Of course, not all optimism is good. Per Haynes , Perry & Stupnisky (2009), over-optimism is actually a risk factor in college. Overly optimistic college students tend to base their optimism on uncontrollable factors, such as innate ability and luck. Think of people who like to repeat self-affirmations or are prone to saying things like “I can do it!” and “I know it will all work out!”, without taking concrete steps to actually make the good thing happen. Time and again, they fall, pick themselves up, repeat their affirmations, then fall again.

Then again, the right kind of optimism is associated with academic success:

Sagone & Caroli (2014): Successful students feel both competent and optimistic, but it’s an optimism based on things under their control – behaviors, knowledge, and tasks – not uncontrollable factors like luck or innate ability. And it’s a reality-based, based mostly on experience overcoming obstacles, completing tasks, and achieving goals. The authors note that prior research had found “significant relationships between locus of control, optimism, and academic success of students. Their own results “demonstrated that the more the university students were likely to personally control the circumstances in their everyday life, the more they expressed a positive self-concept in the present and the future. The more they perceived themselves as academically efficient, the more they judged themselves more positively in the present and the future; in addition, the more the university students were likely to take under their control the everyday life circumstances, the more they perceived themselves as efficient in academic context.”

How about pessimism? Well, pessimism can lead to all sorts of positive outcomes, if you’re the right person in the right context - especially “defensive pessimism”:

Norem and Chang. (2002):  “Defensive pessimism refers to a strategy anxious individuals may use to pursue important goals. These individuals set unrealistically low expectations and then devote considerable energy to mentally playing through or reflecting on all the possible outcomes they can imagine for a given situation.” As a result, defensive pessimists are often good planners and well-prepared for potential barriers to goal achievement. Their self-esteem and life satisfaction also tend to increase over time.

Norem and Chang have the final word here:

“There are potential benefits and costs to both optimism and pessimism…These costs and benefits are often highly sensitive to context and personality. ..Defensive pessimism research makes clear that “one size fits all” prescriptions for optimism and positive thinking do not, in fact, fit some people very well. More generally, one important life outcome that we risk ignoring if we focus exclusively on the positive is the ability to tolerate negative affect and negative self-views while we work toward positive change. Defensive pessimists work through their anxiety on their way toward their goals rather than focusing on increasing their immediate happiness or satisfaction: They remind us that feeling good is not always the highest priority.”

References:

Haynes T.L., Perry R.P., Stupnisky R.H., Daniels L.M. (2009) A Review of Attributional Retraining Treatments: Fostering Engagement and Persistence in Vulnerable College Students. In: Smart J.C. (eds) Higher Education: Handbook of Theory and Research. Higher Education: Handbook of Theory and Research, vol 24. Springer, Dordrecht https://doi.org/10.1007/978-1-4020-9628-0_6

Micahel, L.A.H., Hou, S.-T., and Fan, H.-L. (2011), Creative Self‐Efficacy and Innovative Behavior in a Service Setting: Optimism as a Moderator. The Journal of Creative Behavior, 45: 258-272. doi:10.1002/j.2162-6057.2011.tb01430.x

Nicholls, A. R., R. C. J. Polman, et al. (2008). "Mental toughness, optimism, pessimism, and coping among athletes." Personality and Individual Differences 44(5): 1182-1192. https://doi.org/10.1016/j.paid.2007.11.011

Norem, J.K. and Chang, E.C. (2002), The positive psychology of negative thinking. J. Clin. Psychol., 58: 993-1001. doi:10.1002/jclp.10094

Prati , G. & Pietrantoni, L. (2009) Optimism, Social Support, and Coping Strategies As Factors Contributing to Posttraumatic Growth: A Meta-Analysis, Journal of Loss and Trauma, 14:5, 364-388, DOI: 10.1080/15325020902724271

Sagone, E. and M. E. D. Caroli (2014). "Locus of Control and Academic Self-efficacy in University Students: The Effects of Self-concepts." Procedia - Social and Behavioral Sciences 114: 222-228. https://doi.org/10.1016/j.sbspro.2013.12.689