I have suggested an alternative hypothesis to explain negative effects of optimism on the immune system (see ; Segerstrom, 2001
; Segerstrom et al., 2003
; Solberg Nes et al., in press
). This hypothesis specifies that under difficult circumstances, more optimistic people remain engaged with those circumstances whereas more pessimistic people disengage, avoid, or give up. Giving up can be a physiologically protective response because stressor exposure is minimized in the short term by giving up rather than remaining engaged (although the reverse is true in the long term; Mullen and Suls, 1982
; Suls and Fletcher, 1985
). Therefore, the engagement hypothesis states that when circumstances are easy or straightforward, optimism will be positively related to immunity because engagement can lead to termination of the stressor (e.g., via problem-solving). However, when circumstances are difficult or complex, optimism will be negatively related to immunity because it leads to ongoing engagement with persistent stressors.
Fig. 1 Effects of optimistic engagement and pessimistic disengagement when circumstances are easy or difficult. Effects on immunity shown are drawn from effects on DTH induration in studies of first-year law students (Segerstrom, 2001, 2004).
I have tested this hypothesis in first-year law students, who face two kinds of stress: the difficulty of law school itself, and the difficulties that the time demands of law school create in other domains (Segerstrom, 2001
). First-year students commonly cite the conflict that arises between the time demands of law school and other pursuits, such as social relationships and extramural interests, as one of the most stressful aspects of law school. I take advantage of a natural quasi-experiment that varies the level of this conflict: Moving away to go to law school decreases conflict, whereas staying home makes it worse. For example, an occasional e-mail may maintain one's relationship with an old college roommate from across the country or the state (low conflict with law school), but both parties are likely to be dissatisfied with this level of investment in the relationship from across town (high conflict with law school). Relocation therefore varies the difficulty of circumstances surrounding law school. When students move away from their extramural relationships and commitments to go to law school, their circumstances are relatively easy and straightforward. For them, optimism should have positive effects on immune parameters. When students stay home and have to balance their extramural commitments with law school, their circumstances are relatively difficult and complex. For these students, optimism should have negative effects on immune parameters.
This hypothesis was supported in three independent law school samples. In the first sample, optimism was positively associated with number of CD4+
T cells in peripheral blood when students had moved away to go to law school (b
), but negatively associated when they stayed home (b
= −201). A second sample showed the same effect for delayed-type hypersensitivity (DTH) skin testing with mumps and candida antigens. More optimism was associated with larger mean DTH responses (indicating stronger cellular immunity) when students moved away (b
= 2.4), but smaller DTH responses when they stayed home (b
= −10.1) (Segerstrom, 2001
). A third sample showed the same interaction effect between optimism and relocation on DTH to mumps alone. Again, more optimism was associated with larger DTH responses when students moved away (b
= 3.1) and smaller DTH responses when they stayed home (b
= −4.0) (Segerstrom, 2004
A laboratory study showed a similar interaction between optimism and stressor difficulty: when a difficult, non-responsive laboratory stressor was added to moderate, responsive academic stress, the effect of optimism on DTH also changed (Segerstrom et al., 2003
). Professional students were randomly assigned to either perform a 7 min mental arithmetic task or not. The mental arithmetic task became more difficult with better performance, meaning that individuals engaging the task more fully (e.g., by expending more mental effort) would be “rewarded” with a more difficult task. The DTH antigen was injected after a resting period among those who did not do the task and immediately after the task among those who did. Among participants who did not do the task, more optimism was associated with larger DTH responses 48 h later (b
= 3.4), but among participants who did do the task, more optimism was associated with smaller DTH responses (b
= −5.2). Again, under circumstances when engagement led to greater exposure to a difficult stressor, more optimism was associated a greater decrement in cellular immunity.
Finally, a laboratory study demonstrated that optimists are in fact more likely than pessimists to engage difficult tasks such as those used by Sieber et al. (1992)
and Segerstrom et al. (2003)
, providing a plausible psychological mediator of the immune effects (Solberg Nes et al., in press
). Participants were given a set of difficult anagrams to solve during a 20 min period. Optimistic participants worked longer on the anagrams on their first attempts to solve them, indicating greater task engagement. More importantly, optimists also had higher skin conductance and cortisol after the task, providing plausible physiological mediators of immune effects (e.g., activation of the hypothalamic-pituitary-adrenal axis).
2.1. The role of affect and depression
A series of laboratory and naturalistic studies, then, have demonstrated that optimism has positive effects on cellular immunity when stressors are easy or responsive but negative effects when stressors are difficult or less responsive (see ). Although some evidence suggests that engagement is a more likely mechanism for these effects than disappointment and distress (Helgeson, 2003
; Litt et al., 1992
; Stanton and Snider, 1993
; Solberg Nes et al., in press
), examining the role of affect in these studies further differentiates support for the two hypotheses. In particular, the disappointment hypothesis relies heavily on negative affect as a mediator. For the disappointment hypothesis to be true, optimists have to be “let down” affectively by their failure to realize a positive future. Engagement, by contrast, is a state that can have affective correlates (e.g., excited, involved, interested) but is primarily cognitive and motivational.
Summary of studies: interactions between optimism and stressor type predicted immune parameters
All of these studies, with the exception of Sieber et al. (1992)
, have examined either state or trait negative affect as an explanation for the effects of optimism. In the law student studies, effect sizes stayed the same before and after controlling for positive and negative daily mood (Segerstrom, 2001
). Positive daily mood associated with larger DTH responses and negative daily mood associated with smaller DTH responses (Segerstrom, 2004
), but mood could not account for the optimism effect. These assessments covered the 24 h preceding skin test administration and the 48 h between administration and evaluation, so mood during the skin test was apparently not the mechanism by which optimism affected immune responses. This poses a problem for the disappointment model, which posits that difficult circumstances are more distressing for optimists because their positive expectations have been violated.
The results for trait negative mood were mixed. In one law student sample (Segerstrom, 2001
), controlling for trait negative mood completely accounted for the optimism effect on immunity (and vice versa), but in the Cohen et al. (1999)
sample of community-dwelling women, trait negative mood did not account for any of the effects of optimism on T cells. Similarly, in the mental arithmetic laboratory study, trait negative mood did not account for any of the effects of optimism (Segerstrom et al., 2003
). These different results may arise from the use of different scales to measure trait negative mood. For example, measures vary in the degree to which they contain items that substantively overlap with optimism (e.g., “I'm seldom apprehensive about the future”; Costa and McCrae, 1992
), and therefore the degree to which they would necessarily decrease optimism effects. Importantly, in these studies, trait negative mood cannot account for variance above and beyond the effects of optimism, suggesting that it is the part of trait negative mood that overlaps with optimism that predicted effects on the immune system. One study examined the effects of an alternative trait mediator, conscientiousness. Conscientiousness is closely linked to the engagement model through its emphasis on goal pursuit and achievement striving (e.g., “I work hard to accomplish my goals”; Costa and McCrae, 1992
). In the mental arithmetic study, conscientiousness accounted for most of the optimism effect, supporting the engagement model (Segerstrom et al., 2003
) and suggesting that the effect of optimism was due to greater tenacity and striving in approaching the task on the part of optimistic and conscientious participants.
Another construct that overlaps with optimism is depression, a psychiatric condition that includes problems with affect (e.g., sadness), cognition (e.g., hopelessness), and behavior (e.g., inactivity). Affective mediation having been ruled out, it is still possible that other aspects of depression are active in these studies. For example, inactivity has accounted for reduced lymphocyte proliferation in depressed women (Miller et al., 1999
). However, the naturalistic studies controlled for potential behavioral mediators such as activity, sleep, and substance use, and excluded individuals taking medications (e.g., antidepressants) that could confound results (Cohen et al., 1999
; Segerstrom, 2001
), and quasi-experimental designs further reduce the possibility of confounds (Segerstrom et al., 2003
; Sieber et al., 1992
). Potential overlap with cognitive factors in depression naturally remains, since hopelessness and pessimism—the inverse of optimism—are characteristic of depression. It is possible that these characteristics of depression interact with stressor qualities to predict immune parameters in a manner parallel to optimism, a possibility that should be explored in future research.