A birth-record based Registry is nearing completion of some 8,000 pairs of twins born in Minnesota from 1936 to 1955, plus some 1,200 pairs of male twins born 1971-81. The middle-aged twins were recruited with graded incentives so that ease of recruitment could be measured; it was found that pairs concordant for ease of recruitment were no more similar than discordant pairs in education, socioeconomic status (SES), or a variety of personality and interest factors, ie, that selection bias may not be a problem in research with adult twins when contacts are only by mail. A 50% decrease in neonatal mortality from 1936-55 to 1971-81 was associated with an increase from 3.5 to 4.0 per thousand in the frequency of viable MZ twin births. The broad heritability of SES, educational attainment, fecundity, and risk for divorce ranges from 0.30 to 0.50, although all 4 variables are plainly multifactorial and the latter 2 both involve variance contributed by a second person. Investigators interested in making use of this research resource are invited to submit proposals.