Methods

Participants

The data used in this study (Supplementary Material File) was collected by the means of crowdsourcing throughout Japan. All samples were allocated by gender, as well as 10-year increments of age. With the intention of avoiding ethical issues, question items on blood type characteristics of this study (Table 5 and Appendix A) were extracted from reviewed academic articles (Sato et al., 1991; Watanabe 1994; Yamazaki & Sakamoto, 1991& 1992 – Tables 2-4). These items were checked again at the previously stated crowdsourcing company which has been passed the Japanese privacy mark (JIS Q 15001). The company confirmed that there was no problem with the question items. It provides customers with anonymized data and obtains informed consent from participants (respondents) prior to its surveys.

Instruments

Survey 1 was conducted in 2018 as a preliminary study with a sample size of 1,000. Japanese single individuals with the age of 20-39 were asked to rate a total of 8 items representing 4 blood type (A, B, O, AB) characteristics, each with scores of 1-5 for their personality traits (the larger the number, the more fitting the trait), and scores of 1 to 4 of relationship and their knowledge on the level between blood type and personality (the larger the number, the more related or informed). Survey 2 was conducted in 2019, and covered 2,000 Japanese people ages 20-59, further expanded blood type characteristics to 12 items, and asked the respondents to answer which blood type they thought these 12 items would categorize. In Survey 2, we added an item of marital status, since the age of participants spanned to 59, which meant that married persons were included.
Each participant’s blood type of was determined by self-report, because most Japanese people know their blood type. In Japan, many people know their blood type because, until recently, it had been common practice to test the blood type of newborns. In Survey 1, the dataset included only those who knew their blood type, as previously instructed. Hence, the valid sample size was 1,000. In Survey 2, 141 out of all the 2,000 respondents, or 7.1% of the total, did not know their blood type. Thereby the data of the remaining 1,859 participants with known blood types were used. The distributions of blood types were almost equal to the Japanese average measured by the Japan Red Cross Blood Center; A: 39.1%, B: 21.5%, O: 29.4%, AB: 10.0% (Okubo, 1997).