The critical effect of social grooming costs on structures of social relationships

The number of possible social relationships a single human being can be involved in is limited, and the distribution of strengths of such relationships show significant skew. This skewness suggests that costs and benefits of the social interactions required to bond with others (social grooming) depend on the strength of the social relationships: if it involved uniform costs and benefits, the distribution would not be skew. In this paper, we show that the cost of social grooming increases with the strength of social relationships, and its gradient increases the width and shallowness of these relationships as evident from an analysis of data from six communication systems. We show that narrow and deep social relationships require higher costs per relationship than do wide and shallow ones, using a comparison with a null model where social grooming costs were assumed to be independent of the strength of social relationships. This may be due to increase in communication volumes, such as number of characters and duration of calls, along with an increase in the strength of social relationships. We test this hypothesis using an individual-based simulation where social grooming costs are assumed to increase linearly with the strength of social relationships; this is the simplest assumption. The results of this simulation suggest that the gradient of social grooming costs increases the width and shallowness of social relationships.

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