Differential association between affect and somatic symptoms at the between- and within-individual level.

Abstract

The established between-subjects associations between affect and somatic symptoms have often been interpreted as indicating a causal effect of affect on somatic symptoms, but it is doubtful whether this is valid. In this study, we evaluate the association between positive affect (PA), negative affect (NA), and somatic symptoms at both the between- and within-subject level. Diary data were collected in the context of an online study called 'HowNutsAreTheDutch'. Participants filled out an online questionnaire, three times a day for 30 consecutive days. A mixed linear model was used to test the contemporaneous and lagged associations between affect and somatic symptoms. Five hundred and eighty-six participants (481 females, median age 39.6years [range 18.1-71.4]) were included with a total number of 28,264 completed questionnaires. At the between-subjects level, a positive association between NA and somatic symptoms was found (B=.60, p<.001), whereas the negative association between PA and somatic symptoms was much smaller (B=-.14, p=.062). At the within-subject level, PA (B=-.33, p<.001) was more strongly associated with somatic symptoms than NA (B=.13, p<.001). The lagged analyses showed a negative association between previous-day PA and somatic symptoms (B=-.05, p=.001). The results suggest that NA is more important for differences in symptom levels between subjects, whereas PA is more important for variations in symptom levels within subjects. Moreover, our results suggest that an increase in PA is followed by a decrease in somatic symptoms after 24hr, which suggests a causal effect. Statement of contribution What is already known on this subject? Affect and somatic processes are closely linked. Cross-sectional studies show, for example, that people with higher levels of negative affect tend to report more somatic symptoms. Findings between individuals, though, might camouflage processes at within-individual level, and it might not always be possible to translate findings at the population level to the individual. However, diary studies are upcoming and show more about processes on individual level. What does this study add? Highlights the difference between processes at the within-individual and the between-individual level. Shows the importance of positive affect at individual level in relation to somatic symptoms. Shows the benefits of the use of new techniques in diary studies.