Introduction

Background

Currently, the effect of genetic factors on personality is estimated to be approximately 50% (Ando et al. 2004; Gerra et al., 2000, Keller et al., 2005). The ABO blood group, discovered by Karl Landsteiner in 1901, is a genetically determined and easily measured biological marker. Hence, much research has been conducted not only on health risks, but also on its impact on personality.
The relationship between blood type and personality is studied at an international scale, and the first academic one using statistics a base was conducted in 1927, by Takeji Furukawa, a Japanese educational psychologist (Furukawa, 1927 & 1930; Nawata, 2014; Rogers et al., 2003; Sato et al., 1992; Wu et al., 2005). Yet, the epoch that most shape present-day research is a Japanese book (Nomi, 1971) written by Masahiko Nomi, a Japanese independent researcher, as well as an English book (Nomi et al., 1988) written by his son, Toshitaka Nomi, and Alexander Besher (Nawata, 2014; Rogers et al., 2003; Sato et al., 1992).

Results of Recent Academic Studies

Questionnaire-based personality assessment, frequently used in psychology and consists of answering to multiple questions regarding multiple self-reported personality traits and integrating into several personality factors by statistical processing. In theory, this means that the self-reported answer will either directly or indirectly appear in the result. Although there are many academic studies on the relationship between blood type and personality, the inconsistency among results (Cattell et al., 1964; Cho et al., 2005; Cramer et al., 2002; Flegr et al., 2013; Furukawa, 1927 & 1930; Gupta, 1990; Jogawar, 1983; Kim et al., 2007; Lester et al., 1987; Mao et al., 1991; Nawata, 2014; Rogers et al., 2003; Sharifi et al., 2015; Sato et al., 1992; Shimizu et al., 2011; Wu et al., 2005) has led to the endless academic controversy about whether the relationship is scientifically confirmed. Many studies examined the association between blood type and personality using the “Big Five” personality test (Goldberg, 1990 & 1992), which has been extensively present in contemporary psychology. However, none of these results was certified as consistent (Cho et al., 2005; Cramer et al., 2002; Flegr, et al., 2013; Rogers et al., 2003; Sharifi et al., 2015; Shimizu et al., 2011; Wu et al., 2005). In 2014, a study that analyzed data of large-scale social surveys in Japan and the US was conducted, yet the study over 10,000 participants revealed no meaningful difference (Nawata, 2014).

Self-fulfilling Prophecy

Self-fulfilling prophecy refers to the phenomenon in which a person who believes in a prophecy learns to act in accordance with the prophecy, thereby bringing the prophecy to being. An example that psychologists have studied is astrology (Glick et al., 1986; Eysenck et al., 1982; Snyder, 1984). If a person’s original personality and his/her sign matches, that tendency becomes stronger. Even if the personality and the sign do not initially match, the personality moves toward what has been indicated. In Japan, South Korea, and Taiwan, roughly half of the people feel the relationship between blood type and personality is legitimate (Cho et al., 2005; Kamise et al., 1994 & 1995; Sato et al., 1992; Wu et al., 2005; Yamaoka, 2009). Logically, the self-fulfilling findings among astrology suggest that one’s personality would change in a direction that fits relevant as personality description – of which shown by blood type. Based on this self-fulfilling prophecy hypothesis, several large-scale surveys in Japan which were designed or analyzed by psychologists or other academic researchers after 1990, clearly confirmed the phenomenon (Matsui, 1991; Muto et al., 2012; Ryu et al., 2007; Sakamoto et al., 2004; Yamaoka, 2009; Yamazaki et al., 1991). Archetypical sample sizes of these were 6,660 (Yamaoka, 2009), 11,766 (Matsui, 1991), 32,347 (Sakamoto et al., 2004; Yamazaki et al., 1991 – these two used the same sample) and over 100,000 (Muto et al., 2012; our estimation; the exact number was not specified in this report, although it alluded the size was much larger than preceding ones). Nevertheless, there is not existing coherent statistical difference in respondents without knowledge of astrology or blood type. The current scientific consensus is that these differences are self-fulfilling phenomena induced by the “contamination by knowledge” (Cramer et al., 2002; Eysenck, 1982; Kim et al., 2007; Matsui, 1991; Ryu et al., 2007; Sakamoto et al., 2004; Sato et al., 1992; Yamaoka, 2009; Yamazaki et al., 1991).

Contemporary Trends and Issues

After 2000, a growing number of studies proved the previously questioned link between blood type and physical constitution, with the exception being in the weak gastrointestinal tract: this demonstration proposed a new approach to medicine (Ewald et al., 2016; Risch, 2000). There had also been several studies on biological factors, which investigated whether physical constitution affected personality (Hobgood, 2011a & 2011b). In 2015, a genotype of blood type and the TCI Personality Test had been determined to be related, as predicted by blood type personality theory (Tsuchimine et al., 2015).
It is understood that personality involves complex interactions of genetic factors such as gender and age, and such interactions between these factors are typically non-linear (Kawamoto et al., 2015; Lehmann et al., 2013; Weisberg et al., 2011). However, the majority of the previous studies assumed that the effect of blood type is linear and coherent, regardless of gender or age (Cattell et al., 1964; Cho et al., 2005; Cramer et al., 2002; Flegr et al., 2013; Furukawa, 1927 & 1930; Gupta, 1990; Jogawar, 1983; Kim et al., 2007; Lester et al., 1987; Nawata, 2014; Rogers et al., 2003; Sharifi et al., 2015; Sato et al., 1992; Shimizu et al., 2011; Wu et al., 2005). Statistical methods used to design questionnaire-based personality tests, such as principal component analysis and correlation analysis, basically assume a linear relationship between variables. Therefore, there is no theoretical guarantee that these statistical methods or personality tests will accurately analyze real, non-linear data.