When a self-report instrument includes a balanced number of positively and negatively worded items, factor analysts often use method effect factors to aid model fitting. One of the most widely investigated sources of method effects stems from the respondent tendencies to agree with an item regardless of its content. The nature of these effects, often referred to as acquiescence, is still debated. This study provides a unique contribution to the understanding of the substantive nature of these factors. The revised Life Orientation Test was administered to 653 twins (40% males) to unravel the genetic and the environmental components of method effect associated with positively worded items. Biometric modeling revealed significant heritability for the method effect factor along with strong unique environmental influences. This provides a substantive interpretation of method effects as a stable individual tendency. Theoretical and practical implications of the findings are discussed. (Contains 2 tables and 2 figures.)