There can be little doubt that the selfobject experience of twinship is vital to psychological well-being insofar as a sense of being “a human among humans” tends to mitigate the anguish of existential uncertainty. However, an intense need for twinship in the aftermath of trauma is often the prelude to much pain and suffering. This article attempts to show how trauma-generated twinship needs may lead to denials of sameness and difference and the creation of us–them dichotomies. It proposes that dichotomous gender should be viewed as a trauma-generated relational pattern by means of which unbearable experiences of uncertainty are transformed. Because analysts are as likely to have been traumatized as their patients, the shadow of what I call “the dark side of twinship” falls on many analytic relationships. A clinical example involving a woman whose need for sameness was very great illustrates how struggles over twinship can shape the treatment process.