There have been extensive studies regarding to the Sasang type-specific biopsychosocial features, pathophysiological symptoms, and interventions with diverse expertise. However, most of the previous studies have focused on particular clinical and normal groups with limited variation in gender or age [7, 12, 16], so possible changes with age could not be identified. Therefore, the lifetime pattern of biopsychological changes of each Sasang types was explored in the present large, nationwide study using the SPQ and BMI as reliable measures of biopsychological characteristics of the Sasang typology.
As a first step, the three-factor model of SPQ was examined and found to be acceptable. The correlation analysis between SPQ and BMI showed only weak correlation, confirming that these two aspects of temperament and constitution represent largely independent biopsychological features of Sasang typology .
The SPQ score provided stable temperamental differences that increased in the order of So-Eum, Tae-Eum, and So-Yang Sasang type even when the gender difference was considered. The age trend of total SPQ score in each Sasang type groups was found to be relatively flat, consistent with previous findings about the stability with age of the personality traits of Extraversion  and of Novelty Seeking and Harm Avoidance . This result is consistent with previous studies on the psychological features of Sasang typology [4, 25] that support the rank ordering of SE < TE < SY axis for the total score of SPQ.
It was observed that the SPQ tends to be low in the 30’s and high in the 50’s for men. This may be related to the level of education because 368 (78.6%) participants in their 30’s were college or graduate school graduate, whereas only 121 (24.5%) participants in their 50’s were this highly educated. In addition, with increasing age, the SPQ score for men showed an increasing trend and a decreasing trend for women. This apparent life span change may be influenced by social interaction because variation in Extraversion, Novelty Seeking, and Harm Avoidance can all be influenced by the social environment [8, 26]. However, this possibility should be examined in future studies because multiple biological, educational, and social variables may be confounded in our cross-sectional study of people of varying ages.
The rank order of BMI scores showed an increase from So-Eum, So-Yang, and Tae-Eum type, confirming the distinguishing phenotypic features of each Sasang types in previous studies [2, 12, 19]. These BMI differences between Sasang types were consistently maintained even when gender and age was considered. Such results appear to explain the clinical characteristics of people with the Tae-Eum Sasang type who often have high triglyceride and cholesterol levels , body fat mass , insulin resistance , and high blood pressure . Furthermore, the change in BMI with age is consistent with a previous study of the general population in Korea , which showed BMI decreasing with age for men and increasing with age for women.
An interesting aspect of the present study is that similarities surpassing time and culture can be seen when our results are compared to other studies involving medical typologies across the world [3, 7, 30]. For instance, the three common Sasang types are similar to the Melancholic, Choleric, and Phlegmatic humoral types of Hippocrates and Galen , the Asthenic, Athletic, and Pyknic types of Kretschmer , the Ectomorph, Mesomorph, and Endomorph somatotypes of Sheldon , and Vata, Pitta, and Kapha of Prakriti in Ayurveda .
As for the psychological features, previous research [7, 9, 18] has suggested interesting similarities of various temperament typologies in western psychology with Sasang typology. For example, the sanguine, melancholic, choleric, and phlegmatic types proposed by Galen are remarkable similar to the phenotypic characteristics of Tae-Yang, So-Eum, So-Yang, and Tae-Eum Sasang types, respectively . The psychological theories of Avicenna , Kant , Wundt , Adler , Pavlov, Gray , Eysenck  and Cloninger  were suggested to be related to the temperament types of Hippocrates and Galen [42, 43] from the perspective of the West .
Extraversion and Neuroticism have also been proposed as domains explaining the psychological characteristics in Sasang typology [7, 9, 18]. The relevance of these personality factors could be understood when it is recognized that the name for the four Sasang types is a theoretical combination of two terms, Eum-Yang (Yin-Yang or Passive-Active) and Tae-So (Big-Little or Large-Small), although the name itself may not explain each Sasang types in full .
The Big-Little (Tae-So) concept might come from the Confucianism, which was based on studies of social life. Accordingly, a Big person represents someone who has a more inclusive or bigger personal perspective on society, that is, one who is more generous, caring, forgiving, considerate and selfless as well as someone who has more fully developed higher cognitive functions, and who is thereby more stable, mature, and non-neurotic personality in general [45, 46]. In contrast, the Yin-Yang (Eum-Yang) concept originated in Taoism, which focused on explaining the rules of nature and so has served as a theoretical backbone of traditional medicine for thousands years in the East. The Yin-Yang aspect embodies the two opposing and complementary sides of nature as introvert-extrovert, cold-hot, wet-dry, moon-sun, night-day, dark-bright, femininity-masculinity, fast-slow, soft-solid, active-passive, and so on. The four possible combinations of these two dichotomies might form the basis for the Sasang typology.
Typologies of personality were originally understood in terms of their similarity to variation in seasons of the year, temperature (cold-warm), and humidity (wet-dry), as in the humoral theory of Hippocrates and Galen [39, 47, 48] and in the models of temperament and personality developed by Pavlov, Eysenck, and Gray [39, 42].
The Sasang type of a person seems to be a clinical prototype retaining biopsychological profiles that remain across the life span and help to explain clinical patterns of type-specific pathophysiological symptoms along with type-specific treatment responses [7, 10, 12]. If we are able to investigate and compare the Eastern and the Western perspectives using reliable biopsychosocial methods like the TCI [7, 20], a more integrative outlook on the human nature could be achieved . The TCI is regarded as a well-established personality test measuring psychobiological processes within the person and the maturity of personality development and its implication on health as well as a person-centered and multidimensional profile of neurobiological predispositions which is considered as the foundation of contemporary traditional personalized medicine [20, 49–51].
A number of limitations of the present study should be mentioned. First, this study examined the life span biopsychological changes of each Sasang type with pooled cross-sectional measures. However, these findings should be confirmed in longitudinal studies.
Second, subjects below the age of 19 were not included in current study due to the difficulty of recruitment and absence of standardized measures for child and adolescent subjects . Previous research with the Junior TCI Novelty Seeking scale of children is remarkably similar to that in adults , so more research is warranted in younger subjects.
Third, BMI is a well-established index for international health studies and has shown usefulness in Sasang typology studies with Asian population [2, 12–15]. However, the BMI can be used for the measure of obesity or adiposity, and the epidemic of obesity in the West may make it difficult to distinguish among somatotypes because obesity will obscure differences related to constitutional typologies. We suggest that the circumference ength of neck or chest , Bitragus to Submandibular arc length , height-width ratio of face , basal metabolic rate , and Ponderal Index may be more useful alternatives for the further anthropometric studies.
Last but not the least, the Tae-Yang type subjects were not included in the Sasang type group comparison due to their small number (n = 59). Such infrequency has been observed in many previous studies in which the proportion of that group is supposed to be less than 0.1% . However, the SPQ and BMI of the entire Tae-Yang type group was 29.29 ± 0.80 and 21.15 ± 0.36, respectively, which may be considered that the Tae-Yang type would be physically akin to the So-Eum type , while psychologically to the So-Yang type. The biopsychological features of Tae-Yang type with objective measures should be needed for the future studies.