Background: One of the challenges of psychological research is obtaining a sample

representative of the general population. One largely overlooked participant characteristic is

sub-clinical levels of psychiatric symptoms.

Methods: A series of studies were conducted to assess i) whether typical psychology study

participants had more psychiatric symptoms than the general population, ii) whether there are

sub-groups defined by psychiatric symptoms within the no-diagnosis, no-medication

participant pool, and iii) whether sub-clinical levels of psychiatric symptoms have an effect on

standard behavioural tasks. Five UK national datasets (N > 10,000) were compared to data from

psychology study participants (Study 1: n = 872; Study 2: n = 43,094; Study 3: n = 267).

Results: Psychology study participants showed significantly higher levels of anxiety and

depression, and lower well-being, according to four commonly used mental health measures

(GHQ-12, PHQ-8, WEMWBS, WHO-5). Five sub-groups within the psychology study

participant group were identified based on symptom levels, ranging from none to significant

psychiatric symptoms. These groupings predicted performance on tests of executive function,

including the Stroop task, and the n-back task, as well as measures of intelligence.

Conclusions: This study demonstrates that standard psychology participant pools are

unrepresentative, and suggests that a failure to account for psychiatric symptoms when

recruiting for any psychological study is likely to negatively impact the reproducibility and

generalisability of psychological science.