Background: Little is known about the etiologic relationship between obsessive–compulsive (OC) symptoms and traits of OC personality disorder. The traits include perfectionism and rigidity. Some theorists have proposed that OC personality disorder is one of several disorders falling within an OC spectrum. This implies that OC personality traits and symptoms should have etiologic factors in common, and this should not be simply because symptoms and traits are both shaped by nonspecific etiological influences, such as those shaping negative emotionality (neuroticism). Methods: To investigate these issues, a community sample of 307 pairs of monozygotic and dizygotic adult twins provided scores on six types of OC-related symptoms, two markers of negative emotionality, and a measure of OC personality traits. Results: Analyses indicated that symptoms and traits arose from a combination of genetic and nonshared environmental factors. A matrix of genetic correlations was computed among the variables, which represented the correlations between the genetic components of pairs of variables. A matrix of environmental correlations was similarly computed. Each matrix was factor analyzed. One genetic factor was obtained, indicating that all variables were influenced by a common genetic factor. Three environmental factors were obtained, with salient loadings on either (a) all six OC symptoms, (b) negative emotionality and obsessing, or (c) OC personality traits and ordering. Conclusions: OC symptoms and traits were etiologically related primarily because they are shaped by the same nonspecific genetic factor that influenced negative emotionality. Implications for the concept of the OC spectrum are discussed. Depression and Anxiety, 2011. © 2011 Wiley-Liss, Inc.