letters to nature366 NATURE|VOL 410|15 MARCH 2001|www.nature.com19. Terborgh, J. Five New World Primates (Princeton Univ. Press, Princeton, 1983).20. Leighton, M. Modeling dietary selectivity by Bornean orangutans: evidence for integration of multiplecriteria in fruit selection. Int. J. Primatol. 14, 257±313 (1993).21. Gautier-Hion, A. et al. Fruit characters as a basis of fruit choice and seed dispersal in a tropical forestvertebrate community. Oecologia 65, 324±337 (1985).22. Davies, A. G. & Oates, J. F. in Colobine Monkeys (eds Davies, A. G. & Oates, J. F.) 229±249 (CambridgeUniv. Press, Cambridge, 1994).23. Wrangham, R. W., Conklin-Brittain, N. L., & Hunt, K. D. Dietary responses of chimpanzees andcercopithecines to seasonal variation in fruit abundance: I. antifeedants. Int. J. Primatol. 19, 949±970(1998).24. Onishi, A. et al. Dichromatism in macaque monkeys. Nature 402, 139±140 (1999).25. Struhsaker, T. T. Ecology of an African Rainforest (Univ. Florida Press, Gainesville, 1997).26. Lucas, P. W. et al. Fieldkit to characterize the physical, chemical, and spatial aspects of potentialprimate foods. Folia Primatol. 72, 11±25 (2001).27. Osorio, D. & Vorobyev, M. Colour vision as an adaptation to frugivory in primates. Proc. R. Soc. Lond.B 263, 593±599 (1996).28. Darvell, B. W., Lee, P. K. D., Yuen, T. D. B., Lucas, P. W. Meas. Sci. Technol. 7, 954±962 (1996).29. Newton-Fisher, N. E. The diet of chimpanzees in the Budongo Forest Reserve, Uganda. Afr. J. Ecol. 37,344±354 (1999).30. Gartlan, J. S., McKey, D. B., Waterman, P. G., Mbi, C. N. & Strusaker, T. T. A comparative study of thephytochemistry of two African rainforests. Biochem. Syst. Ecol. 8, 401±422 (1980).AcknowledgementsWe thank D. Osorio for help with colour registration; E. Ting, P. Y. Cheng, I. C. Bruce,R. T. Corlett, L. Ramsden, N. Yamashita and A. Walker for comments, P. Kagoro,B. Balyeganira and M. Musana for ®eld assistance in Uganda; J. Magnay, R. W. Wranghamand C. A. Chapman for logistic support in Uganda; and the Ugandan National Council forScience and Technology, Ugandan Wildlife Authority and Makerere University BiologicalField Station for permission to work at Kibale. Supported by Research Grants Council ofHong Kong, National Geographic Society, Sigma Xi, Explorer's Club and CroucherFoundation of Hong Kong.Correspondence and requests for materials should be addressed to N.J.D.(e-mail: [email protected])..................................................................Suppressing unwanted memories byexecutive controlMichael C. Anderson & Collin GreenDepartment of Psychology, University of Oregon, Eugene, Oregon 97403-1227,USA..............................................................................................................................................Freud proposed that unwanted memories can be forgotten bypushing them into the unconscious, a process called repression1.The existence of repression has remained controversial for morethan a century, in part because of its strong coupling with trauma,and the ethical and practical dif®culties of studying such pro-cesses in controlled experiments. However, behavioural andneurobiological research on memory and attention shows thatpeople have executive control processes directed at minimizingperceptual distraction2,3, overcoming interference during shortand long-term memory tasks3±7and stopping strong habitualresponses to stimuli8±13. Here we show that these mechanismscan be recruited to prevent unwanted declarative memories fromentering awareness, and that this cognitive act has enduringconsequences for the rejected memories. When people encountercues that remind them of an unwanted memory and they con-sistently try to prevent awareness of it, the later recall of therejected memory becomes more dif®cult. The forgetting increaseswith the number of times the memory is avoided, resists incen-tives for accurate recall and is caused by processes that suppressthe memory itself. These results show that executive controlprocesses not uniquely tied to trauma may provide a viablemodel for repression.Executive control processes studied in behavioural6,9,14andneurobiological2,4,10±13,15±17research on cognition may provide amechanism for the voluntary form of repression (suppression)proposed by Freud1. To test this hypothesis, we adapted the go/no-go paradigm used to study executive control over motor actionsin primates18and humans15±17for use in a memory retrieval task.First, we trained subjects on 40 unrelated word pairs (for example,ordeal±roach) so that they could recall the right-hand member ofeach pair when provided with the left-hand member. Next, subjectsperformed a critical task requiring them to exert executive controlover the retrieval process. On each trial of this think/no-think task, acue from one of the pairs appeared on the computer screen.Depending on which cue appeared, subjects were told either torecall and say (think about) the associated response word (respondpairs), or not to think about the response (suppression pairs). Forthe latter pairs, we emphasized that subjects should not allow theassociated memory to enter consciousness at all. If subjects acci-dentally responded to a suppression pair, they heard a beep signal-ling an error. To increase the need to recruit inhibitory controlmechanisms, we required subjects to ®xate on the cue word for theentire time (4 s) that it appeared on the screen, discouragingperceptual avoidance and generating a constant threat that theassociated memory might intrude into consciousness. Thus, sup-pression trials required the stopping of both a prepotent motor6575859501816Number of repetitionsPer cent recalled SuppressResponda6575859501816Number of repetitionsPer cent recalledSuppressRespond6575859501816Number of repetitionsPer cent recalledSuppressRespond6575859501816Number of repetitionsPer cent recalledSuppressRespondd6575859501816Number of repetitionsPer cent recalledSuppressResponde6575859501816Number of repetitionsPer cent recalledSuppressRespondf6575859501816Number of repetitionsPer cent recalledRespond-sameRespond-indepSupp-sameSupp-indepgbcSame probe Independent probeFigure 1 Final recall for respond and suppression items as a function of the number ofrepetitions for the same-probe (SP) and independent-probe (IP) tests. a, b, Experiment 1;c, d, experiment 2; e, f, experiment 3; g, averaged across experiments. Note the negativeslope for recall of the suppressed item, indicating
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