Some of these studies were again open to concerns about the absence of operational diagnostic criteria and prompted a reassessment, applying operational criteria of varying degrees of stringency by Gottesman and Shields (1972). Criteria that focused solely upon a limited range of positive symptoms (Schneider's "first-rank" symptoms; see Chapter 1) gave no evidence of genetic contribution, whereas applying broader criteria based upon DSM-III, the estimate of heritability rises to a figure approaching 80% (Farmer, McGuffin, & Gottesman, 1987).