Perceptual and Motor Skills, 2004,98, 141-146. O Perceptual and Motor Skills 2004 SELF-CONTROL IN RHESUS MACAQUES (MACACA MULATTA): CONTROLLING FOR DIFFERENTIAL STIMULUS EXPOSURE ' ALLEN D. SZALDA-PETREE, BAINE B. CRAFT, LORI M. MARTIN, AND HEIDE K. DEDITIUS-ISLAND Department of Psychology The University of Montana-Missoula Summary.-Previous research on self-control using macaques (Macaca fascicularis) showed these animals have a strong bias for a delayed, larger reinforcer (Self-control) over an immediate, smaller reinforcer (Impulsive). Typical studies of self-control have used a discrete trials methodology with a secondary discriminative stimulus during the delay periods. This results in a greater exposure to the stimulus representing the self- controlled option and may account for some of the early exclusive preference for self- control observed. The present experiment examined self-control bias in three rhesus macaques (Macaca mulatta) while controlling for differential durations of stimulus ex- posure. Subjects were presented stimuli via a computer monitor and made choices by touching the stimulus at which point both stimuli were removed for the delay periods. AU three subjects displayed a nearly exclusive bias for the delayed, larger reinforcer (self-control). These results are consistent with previous studies, despite the variations in methodology and species. Self-control is defined as the choice of a larger but delayed reinforcer over a smaller and immediately available reinforcer while the opposite has been defined as impulsive (Rachlin & Green, 1972; Ainslie, 1974; Logue, 1998). A relatively wide literature exists on various factors that influence self-controlled and impulsive choice in humans (Mischel, Ebbesen, & Zeiss, 1972; Millar & Navarick, 1984; Mischel, Shoda, & Rodriguez, 1989; Logue, Forzano, & Tobin, 1992; Forzano & Logue, 1994; Logue, 1998) and pigeons (Rachlin & Green, 1972; Ainslie, 1974; Mazur & Logue, 1978; Grosch & Neuringer, 1981; Logue, Rodriquez, Pena-Correal, & Mauro, 1984; Chelo- nis, King, Logue, & Tobin, 1994; Mazur, 1994). Overall, results from stud- ies using human subjects have indicated general bias towards self-control. This self-control bias signifies that choice is largely controlled by the relative magnitude of the reward. In contrast, pigeons have a strong impulsive bias, signifying that choice is controlled by the relative delay to reward. While the literature investigating self-control in other nonhuman animal species is lim- ited, e.g., squirrel monkeys, long-tailed macaques, pigeons, rats, those report- ed tend to show an impulsive choice bias similar to that observed with 'Address enquiries to A. D. Szalda-Petree, Department of Psychology, Skaggs Building, Room 143, The University of Montana-Missoula, Missoula, MT 59812-MPSI01 or e-mail (allen.szalda- petree@umontana.edu).