In this chapter the authors present some pretty convincing evidence indicating that attachment (as measured using the Experience of Close Relationships Scale but probably other romantic attachment scales as well) is fundamentally dimensional rather than typological.

A useful summary of the paper has been provided here on the website of the Adult Attachment Lab, University of California at Davis.

Fraley went on to apply the same method to observations of behaviours in the Strange Situation Procedure in Fraley & Spieker (2003)