Are the effects of psychosocial exposures attributable to confounding? Evidence from a prospective observational study on psychological stress and mortality

Study objectives-To examine the association between perceived psychological stress and cause specific mortality in a population where perceived stress was not associated with material disadvantage. Design-Prospective observational study with follow up of 21 years and repea t screening of half the cohort five years from baseline. Measures included perceived psychological stress, coronary risk factors, and indices of lifec ourse socioeconomic position. Setting-27 workplaces in Scotland. Participants-5388 men (mean age 48 years), at first screening and 2595 men at second screening who had complete data on all measures. Main outcome measures-Hazard ratios for all cause mortality and mortality f rom cardiovascular disease (ICD9 390-459), coronary heart disease (ICD9 410 -414), smoking related cancers (ICD9 140, 141, 143-9, 150, 157, 160-163, 18 8 and 189), other cancers (ICD9 140-208 other than smoking related), stroke (ICD9 430-438), respiratory diseases (ICD9 460-519) and alcohol related ca uses (ICD9 141, 143-6, 148-9, 150, 155, 161, 291, 303, 571 and 800-998). Results-At first screening behavioural risk (higher smoking and alcohol con sumption, lower exercise) was positively associated with stress. This relat ion was less apparent at second screening. Higher stress at first screening showed an apparent protective relation with all cause mortality and with m ost categories of cause specific mortality. In general, these estimates wer e attenuated on adjustment for social position. This pattern was also seen in relation to cumulative stress at first and second screening and with str ess that increased between first and second screening, The pattern was most striking with regard to smoking related cancers: relative risk high compar ed with low stress at first screening, age adjusted 0.64 (95% CI 0.42, 0.96 ), p for trend 0.016, fully adjusted 0.69 (95% CI 0.45, 1.06), p for trend 0.10; high compared with low cumulative stress, age adjusted 0.69 (95% CI 0 .44, 1.09), p for trend 0.12, fully adjusted 0.76 (95% CI 0.48, 1.21), p fo r trend 0.25; increased compared with decreased stress, age adjusted 0.65 ( 95% CI 0.40, 1.06), p for trend 0.09, fully adjusted 0.65 (95% CI 0.40,1.06 ), p for trend 0.08. Conclusions-This implausible protective relation between higher levels of s tress, which were associated with increased smoking, and mortality from smo king related cancers, was probably a product of confounding. Plausible repo rted associations between psychosocial exposures and disease, in population s where such exposures are associated with material disadvantage, may be si milarly produced by confounding, and of no causal significance.