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).