  Suicide explains the largest number of death tolls among Japanese adolescents
in their twenties and thirties. Suicide is also a major cause of death for
adolescents in many other countries. Although social isolation has been
implicated to influence the tendency to suicidal behavior, the impact of social
isolation on suicide in the context of explicit social networks of individuals
is scarcely explored. To address this question, we examined a large data set
obtained from a social networking service dominant in Japan. The social network
is composed of a set of friendship ties between pairs of users created by
mutual endorsement. We carried out the logistic regression to identify users'
characteristics, both related and unrelated to social networks, which
contribute to suicide ideation. We defined suicide ideation of a user as the
membership to at least one active user-defined community related to suicide. We
found that the number of communities to which a user belongs to, the
intransitivity (i.e., paucity of triangles including the user), and the
fraction of suicidal neighbors in the social network, contributed the most to
suicide ideation in this order. Other characteristics including the age and
gender contributed little to suicide ideation. We also found qualitatively the
same results for depressive symptoms.
