'In early 1979, Yufe’s then-wife, Ona, showed him a magazine article
  about the “Jim Twins,” a pair of long-separated Ohio twins who were
  each named Jim by their respective adoptive parents. Like Yufe and his
  brother, Jim Springer and Jim Lewis found each other as adults and were
  astounded by the parallels in their lives, including similar jobs in
  law enforcement and ex-wives with the same first name.

  The two Jims had become the first subjects of the Minnesota twins
  study. Yufe was intrigued and thought he and Oskar should also
  participate.

  ...

  The researchers jumped at the chance and invited them to Minnesota for a
  week. Yufe and Stohr became the seventh set of twins to enter the study.'

The Study was instantiated when a potentially unusual situation occurred. People were then allowed to join the study themselves, after hearing about strange twin coincidences.

This was not a study of a random sample of twins.