It can be tricky to discuss the pessimism bias without also considering the optimism bias. Both biases can indeed coexist in a population as cognitive phenomena since not every bias applies to every person. Moreover, both biases can also coexist for the same person, depending on the context of the situation (more on that later). In general, however, most people skew towards expressing an optimism bias. For example, the majority of people expect to live longer and be healthier than others.4
The neural mechanisms that are suggested to be involved in mediating optimism include a functional connectivity between the rostral anterior cingulate cortex and the amygdala. This network is also related to pessimism; specifically, irregularities in this pathway are linked to depression,5 which in turn often feeds into pessimism.6 This divergence from the typical circuitry involved in optimism can lead to negative expectations, such as living shorter lives and being less healthy than others.
Aside from neuroscientific evidence, cultural differences in optimism and pessimism biases have been recorded. One study found that Westerners (European American respondents) were more likely to expect positive events to happen to them compared to Easterners (Japanese respondents), with Easterners being more likely to expect negative events to happen to them.7 The authors of the study suggest the results may be due to the idea that Western individualism promotes self-enhancement while Eastern collectivism promotes self-criticism, with these cultural connotations manifesting in our cognition.
The biological aspect of aberrant neural circuitry in the mechanisms of optimism, coupled with a cultural view on the optimism-pessimism dichotomy, bring nuance to the subject and pose great challenges in untangling a causal relationship that can explain the pessimism bias. Does pessimism cause depression, or does depression cause pessimism? How does one’s culture factor in? One study looking at 852 twin pairs found distinct genetic influences on optimism and pessimism in addition to a family-level environment effect.8 Given the nebulous nature of pessimism, there is likely no single clear-cut variable that can summarize the phenomenon, but rather a multitude of contributing factors.