Introduction

The present psychometric investigation, which is a part of a psychiatric follow-up study of unselected cases o f closed head injury among monozygotic twins, proposes to study the effect of head injury, particularly the effect on cognitive functions. The co-twins were used as controls, mainly in order to keep constitutional factors as much as possible alike ,in the subjects and controls. Using a method of this kind p u t a limit on the number of subjects that could be studied but it meant that their performance could be analyzed in greater detail and involved a new approach to the problems under discussion. A cli.nica1 study of the saline series will be taken up in a separate communication by one of us (D. 1955). During the years 1920 to and including 1953, altogether 14,647 persons were admitted for head injury to the eleven hospitals in Scania in the south of Sweden. All the twins with an acute injury were sorted out from these 14,647 persons by consulting the special index for all the twins born in Scania which we have at the Psychiatric Department. This gave 36 monozygotic twin probands still alive and with living co-twins. In order to prevent the results of the various groups from 11ein.g too much distorted by very old o r very young persons, we decided only to examine twins between 10 and 60 years of age, which meant excluding 4 of the 36 pairs. One pair refused to be tested, thus leaving 31 pairs. In 3 pairs both had had head injuries and these 3 were excluded when the probands and partners were compared. On the average ten years had elapsed between the time the subject received the head injury and the time he was examined by us, and the shortest interval amounted to three years. No neurosurgical measures had been used in any of the tested cases. None of the injuries were associated with signs of intra-