Two types of social grooming methods depending on the trade-off between the number and strength of social relationships.

Humans use various social bonding methods known as social grooming, e.g. face to face communication, greetings, phone and social networking sites (SNS). SNS have drastically decreased time and distance constraints of social grooming. In this paper, I show that two types of social grooming (elaborate social grooming and lightweight social grooming) were discovered in a model constructed by 13 communication datasets including face to face, SNS and Chacma baboons. The separation of social grooming methods is caused by a difference in the trade-off between the number and strength of social relationships. The trade-off of elaborate social grooming is weaker than the trade-off of lightweight social grooming. On the other hand, the time and effort of elaborate methods are higher than those of lightweight methods. Additionally, my model connects social grooming behaviour and social relationship forms with these trade-offs. By analysing the model, I show that individuals tend to use elaborate social grooming to reinforce a few close relationships (e.g. face to face and Chacma baboons). By contrast, people tend to use lightweight social grooming to maintain many weak relationships (e.g. SNS). Humans with lightweight methods who live in significantly complex societies use various types of social grooming to effectively construct social relationships.