Higher-order propositional attitudes (HOPAs), such as "I think that you think that…," figure in many fields including theory of mind, cognitive ethology, and psycholinguistics. Analysis of many examples suggests there may be differences in understandability of HOPAs depending on the type and number of constituents and the presence of recursions. Empirical work on normal adult ability with HOPAs has been lacking, leaving research with special populations without a standard for comparison. An experiment explored the effects of varying the number of individuals in HOPA sentences, up to the eighth order. Significant differences in understandability of HOPA sentences were found between three groups, those that are about (i) oneself, (ii) dyads, and (iii) series of different individuals.