Previous international comparative studies have shown that Americans and other North Americans have a stronger passion for romantic relationships than East Asians such as Japanese.However, this phenomenon cannot be explained by the existing cultural psychological theories that "North Americans are individualistic and independent" and "East Asians are groupist and interpersonal-centered".

　Therefore, this research group considered that the cause of such cultural differences in passion lies in the degree of freedom in choosing lovers in each cultural area, that is, the difference in "relationship fluidity."In a competitive society, such as North American society, where romantic relationships are highly fluid and always compete for attractive romance partners, people are more likely to be worried that their lover will switch to a more attractive opposite sex.At such times, passionate love helps to eliminate the anxiety of the other person through devoted actions to the other person and maintain a stable romantic relationship.

　In this study, we conducted an Internet survey and asked Japanese and Americans about the strength of their passion for their lovers and the degree of freedom in choosing a romantic relationship.Then, when comparing the average values ​​of Japanese and Americans, it was found that Americans feel a stronger passion for their lovers than Japanese.It was also clarified that these social differences in passion are statistically explained by the greater freedom of choice in romantic relationships in American society than in Japanese society.

　This is the first achievement that empirically shows that the strength of the basic emotions that support human reproductive behavior, the passion in romantic relationships, depends on the characteristics of the society surrounding people.

Paper information:[Evolutionary Psychology] Passion, Relational Mobility, and Proof of Commitment: A Comparative Socio–Ecological Analysis of an Adaptive Emotion in a Sexual Market