A new method of cross-sectional twin sampling from a population may be used in countries where authorities utilize electronic data processing of National Register data.

In Denmark, all National Register data are collected on magnetic tape. In January 1969 it was consequently possible, by the help of a 360-COBOL programme, to select all individuals in the Copenhagen area with identical birthday, month and year, and identical address. The selection was limited to the years 1943 to 1968.

Data from every person were tested against data from all other persons, to prove whether the person had one or more partners with identical data, i. e., twin, triplet, or quadruplet partners. Names and current addresses of the twins were written on self-adhesive labels intended for later correspondence. Had it been desirable, the computer might have produced, in the same operation, index cards and questionnaires ready for mailing. In the primary sample, a few “false” twin pairs from great collective housekeepings (as, for example, colleges) were included. These “false” pairs were excluded after comparing the partners surnames.

From the total population of 1 712 000 people, 4595 twin, 25 triplet, and 1 quadruplet sets were found. As many as 3754 twin pairs were born in 1951-1967. Of these, 34.68% were of opposite sex. Of the remaining pairs, 642 were born in 1943-1950, 28.19% being of opposite sex.