The famous 15-minute expressive-writing study did not measure immunity; a later 20-minute replication found immune-marker changes at six weeks, while a 30-trial meta-analysis found no overall health benefit

Blood was collected the day before writing began, after the final session, and again six weeks later. Pennebaker’s own 2018 review of expressive writing research describes both replications and failures to replicate. In a randomised study of 116 healthy young adults, four 20-minute writing sessions produced no overall improvement in anxiety, depression or physical symptoms. Four 20-minute sessions were followed by changes in specific lymphocyte assays at six weeks in a small 1988 study. Larger bodies of research later found small, variable or null average health effects.