Have you heard of the name "serotonin"?
This is one of the neurotransmitters that have a gread influence on personality.
Neurotransmitters are substances that mediate signaling from neurons (nerve cells) to another.
There are three kinds of genes (LL, LS, SS) of "serotonin transporter", which is closely related to the activity of serotonin, and many studies have confirmed that differences in personality depend on the kind.
Recently, when comparing the degree of the gene influence, some simple "questionnaire survey" seems to show larger differences than psychology "personality tests."
In other words, to investigate the influence of a gene, researchers might fail if stick to the framework of psychology, as mentioned before.
The following is an excerpt from Mr. Masayuki Kanazawa's "Why are type B women so popular?"
1. Questionnaire survey
London School of Economics and Political Science
2. Personality test
Lesch KP, Bengel D et al., Association of anxiety-related traits with a polymorphism in the serotonin transporter gene regulatory region, Science. 1996 Nov 29; 274 (5292): 1527-31.
-> Psychological personality test (NEO-PI-R SD=0.29); effect size [magnitude of influence]: small to medium (d=0.29)
I read another report written by a female Japanese researcher (*1). She used a psychology personality test, called TCI. This time, too, differences are smaller than usual. The result "partial η2 = 0.010" means a small effect: 1% (i. e. effect size [magnitude of influence] is small).
*1 Shoko Tsuchimine, Junji Saruwatari, Ayako Kaneda, Norio Yasui-Furukori (2015)
ABO Blood Type and Personality Traits in Healthy Japanese Subjects
(Excerpts) The MANCOVA data showed a significant difference in TCI scores among the ABO genotype groups (F [7, 1393] = 3.354, p = 0.001). A subsequent univariate analysis showed a significant difference in the mean scores for Persistence among the genotype groups (F = 2.680, partial η2 = 0.010, p = 0.020).