Grief intensity following the loss of a twin and other relatives: test of kinship genetic hypotheses.

Following Darwin, we summarize evidence that grief may be an evolved adaptive mechanism with complex physiological and psychological correlates. Given this state of affairs, kinship genetic theory suggests that in humans ratings of grief intensity should increase with increasing genetic relatedness to the deceased. A study of the bereavement experiences of 49 surviving monozygotic (MZ) and 19 surviving dizygotic (DZ) co-twins provides informative tests of this hypothesis. Participants completed a comprehensive Twin Loss Survey, which was designed to assess intensity of bereavement in response to the death of the twin and other relatives. The mean grief intensity rating for the deceased twin was significantly higher for surviving MZ twins than for surviving DZ twins (p < 0.01). Mean grief intensity ratings for twins significantly exceeded those for mothers (p < 0.05), fathers, grandfathers, grandmothers, and other relatives (p < 0.01). These findings are consistent with kinship genetic predictions.