Discussion

Issues with Personality Tests

Out of all 20 blood type characteristic items, 17 of them (14 after the Bonferroni’s correction) clearly demonstrated the differences as predicted. This study measured self-reported personality, and since many Japanese believe in the relationship between blood type and personality, conventional personality psychology theory suggests the differences among the blood types will certainly appear, as prior mentioned. For the same reason, personality psychology hardly explains that no difference in blood type, or inconsistent results, appeared theoretically (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).
Sung Il Ryu et al. reanalyzed So Hyun Cho et al’s result of the Big Five test (Cho et al. 2005) and found statistically significant differences which match with blood type characteristics in 10 individual items (Ryu et al., 2007). This means that in the case of a “personality factor” composed of multiple items, the difference by the blood type decreases ‒ few significant differences appear.
According to ANOVA results of this study (Appendix C), some items were influenced by age, gender, etc. than blood type, therefore consistent results cannot be obtained without considering these elements in a personality factor. This corresponds to the inconsistent results of many preceding studies conducted by psychologists.
In addition, the reason why psychological personality tests did not show differences in blood type (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) was probably due to the halo effect (Kamise et al., 1994). Differences by the blood type are heavily depended upon “original” words. In many cases, differences did not appear if “similar” words were selected (Sakamoto et al., 2004; Yamaoka, 2009; Yamazaki et al., 1991). For example, Analyses 2 and 3 in Survey 2, respondents identified “cheerful”, “expressive and delicate”, “friendly and sociable”, and “good and humane” in Q1 as characteristics of type O or A, but their actual answers did not yield any statistically significant differences. It should be noted that these items were expected to procure differences, since they were chosen as typical characteristics of blood type in previous surveys by psychologists (Watanabe, 1994).

Consistency with Preceding Studies

Preceding research by psychologists had concluded that there appeared no difference in personality among blood types, and that even if there was a difference predicted by the blood type, it was assumed to be the result of self-fulfillment prophecy phenomena (Kim et al., 2007; Sakamoto et al., 2004; Yamaoka, 2009; Yamazaki et al., 1991). However, backed on this study’s data, it was highly likely that these phenomena occurred because gender and age were not taken into account. The experimental blood type prediction by AI (Amazon Machine Learning) in this study found that adding non-blood type variables, such as gender or age, to the training data considerably increased the accuracy (Appendix D). Moreover, when performing blood type prediction, AI sometimes failed to build its machine learning models, if gender or age of the training and prediction data were different. In this respect, AI technology may suggest that factors such as gender and age affect the characteristics of blood type. Therefore, gender, age, and other factors may offer a better explanation, even if past data were inconsistent.
The personality factors of the Big Five personality test are designed primarily to maximize the variances of respondents (Goldberg, 1990 & 1992). In general, the effect of blood type is smaller than that of gender or age (Nawata, 2014; Yamazaki et al., 1991). For this reason, differences in blood type, which appear in a single item, decrease significantly in personality factors if the gender or the age of the sample is not appropriately controlled. According to a study which compared the effect of blood type and age, the latter was approximately 4 times larger than the former (Yamazaki et al., 1991). Common statistical tests, such as the F -test, compare the variances of the raw data squared, so that the actual values compared are 4 squared, or 16 times. Thus, statistical tests are less likely to show significance. Given these large differences of influence and the non-linear nature of the actual effects, it can be very difficult to detect the effect of blood type with these personality tests. This offers an explanation to the inconsistent results (or no difference) in the actual Big Five personality test. The current personality psychology theory can explain the phenomenon without contradictions.

Individual Differences in Personality Sensitivity

Taste and visual sensitivities vary among individuals, thereby sensory test (e.g., ISO 20613) considers this a self-evident assumption. By the same reasoning, personality sensitivity may vary from person to person, but psychologists rarely take this into account when designing common personality tests, such as the Big Five test (Goldberg, 1990 & 1992).
In Analysis 3, the average score of characteristics that did not fit their blood type in the “I know well”, “I know some” and “I know a little” groups had higher scores (on Survey 1, 3.443, 3.246 and 3.046, respectively) than the average score 2.986 of “no-knowledge” group. Survey 2 showed the same results. If the personality sensitivities were the same among these groups, the average score for non-blood-type-matching characteristics would be the same in each group, but this was not the case. In addition, the average scores of characteristics that “did not fit” his/her blood type in the “I know well” and “I know some” groups were higher than the average score for characteristics that “fit” his/her blood type in the no-knowledge group (3.062).
To explain the results obtained in this study, it is natural to attribute the small differences in the blood type between the two no-knowledge groups, not to the conventionally assumed self-fulfilling prophecy, but to their low sensitivity to their personalities. In Analysis 3, the no-knowledge groups displayed a tendency to be little interested in others and personality. The two groups seemed to unsociable, compared with the other groups, which was also consistent with the results of this study.

Occupation by Blood Type

Hans Eysenck and David Nias suggested that researchers should focus on occupations people select, rather than subjective self-assessments, to avoid the “contamination by knowledge” that arises in the case of astrology. They cited several examples in their book. However, the results were generally in negative (Eysenck et al., 1982).
The situation of blood type and personality is different, unlike astrology. Several academic studies had examined the relationship between blood type and occupation; significant differences were reported especially among politicians and athletes. For example, Masao Ohmura et al, a Japanese psychologist group said, “many Japanese prime ministers are type O” ‒ statistically significant (Ohmura et al., 2013). Another example: Japan has many type O and AB foreign ministers and O defense ministers, which demonstrates statistical significance. In general, athletes tend to be type O. An Italian study said that type O are good athletes of long-distance running (Lippi et al., 2017). In both Japan and South Korea, type O is the best batter in professional baseball. Type B, considered to be at his/her own pace, is supposedly a better player in individual competition than in group. In Japan, there are far fewer B’s among athletes of the FIFA World Cup and female artistic swimmers, which are group games. On the contrary, B is prominent among the talented Japanese professional baseball batters and the professional golfers. Thus, in occupations, there lies distinct statistically significant differences predicted by blood type. These can be recognized as evidence that blood type is related to physical constitution and personality (Kanazawa, 2019).