The healthy control subject in psychiatric research: impulsiveness and volunteer bias.

Exciting and demanding biomedical experiments may attract a specific subgroup of people as volunteers. In the present study of selection bias, subjects volunteering in a psychobiological study that included a potentially painful procedure (lumbar puncture) were compared with those who declined to participate, with regard to scores on personality scales administered during a previous investigation of the same subjects. Significant differences were found on the Eysenck Personality Questionnaire and Karolinska Scales of Personality Impulsiveness scale, suggesting an over-representation of impulsive individuals among the volunteers. If the specific subject of investigation has implications for the type of individual who will participate as a healthy volunteer in biomedical research, variation will be introduced, affecting the independent variable, and the conclusions that can be drawn from such research may be questionable.