Human family and twin studies have established considerable heritable components influencing individual differences in personality traits as assessed by self-report questionnaires. We have investigated a trinucleotide repeat polymorphism in the androgen receptor gene and personality traits. Healthy Swedish subjects (n = 335) were assessed with the Karolinska Scales of Personality inventory. There were tendencies (P > or = 0.006) in some scales indicating possible relationships between the androgen receptor allele length and personality traits related to dominance and aggression. However, after correction for multiple testing, no significant differences were found. We conclude that no significant association could be found between the androgen receptor polymorphism investigated and any personality trait, although the tendencies found are worthwhile subjects for replication attempts.

Human family and twin studies have established considerable heritable components in personality traits as assessed by self-report questionnaires. Recently, an association between a functional polymorphism in the upstream regulatory region of the serotonin transporter gene and neuroticism-related personality traits was reported. Two different serotonin transporter polymorphisms including the previously associated variant were genotyped in two samples of healthy Swedish subjects (n = 127 and n = 178, respectively) assessed with the Karolinska Scales of Personality (KSP) inventory. No statistically significant association between serotonin transporter polymorphisms and any of the eight neuroticism-related KSP scales was found. Thus, the previously reported association between serotonin transporter alleles and neuroticism-related personality traits could not be replicated in the present study.

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.

The present study does not support that the investigated NOTCH4 variants have a major influence on susceptibility to schizophrenia or related neurobiological traits.