The boundary conditions for Bohr’ s law: when is reacting faster than acting? Yaïr Pinto & Marte Otten & Michael A. Cohen & Jeremy M. Wolfe & Todd S. Horowitz Published online: 10 November 2010 # Psychonomic Society, Inc. 2010 Abstract In gunfights in Western movies, the hero typi- cally wins, even though the villain draws first. Niels Bohr (Gamow, The great physicists from Galileo to Einstein. Chapter: The law of quantum, 1988) suggested that this reflected a psychophysical law, rather than a dramatic conceit. He hypothesized that reacting is faster than acting. Welchman, Stanley, Schomers, Miall, and Bülthoff (Proceedings of the Royal Society of London B: Biological Sciences, 277, 1667–1674, 2010) provided empirical evidence supporting “Bohr’ s law,” showing that the time to complete simple manual actions was shorter when reacting than when initiating an action. Here we probe the limits of this effect. In three experiments, participants performed a simple manual action, which could either be self-initiated or executed following an external visual trigger. Inter-button time was reliably faster when the action was externally triggered. However, the effect disappeared for the second step in a two-step action. Furthermore, the effect reversed when a choice between two actions had to be made. Reacting is faster than acting, but only for simple, ballistic actions. Keywords Motor control . Reactive actions . Self-initiated actions . Supplementary motor area . Speed of motion execution The great quantum physicist Niels Bohr also displayed a keen intuition for psychophysics. Bohr was a known aficionado of Hollywood westerns. George Gamow (1988, pp. 237–238) recounts an episode where Bohr watched a movie with several friends, and the group pondered the question of why the hero always wins gun duels, despite the fact that the villain inevitably acted first. Aside from dramatic necessity, Bohr asserted that this phenomenon reflected a fundamental law of human behavior: reacting is faster than acting. Since the villain draws first, he must initiate his own motion, thereby introducing thoughts and doubts into his action. The hero’ s action, meanwhile, is triggered by an external cue and is therefore not affected by these distracting thoughts. Gamow reports that Bohr and his friends tested the hypothesis empirically with toy guns the next day, but the anecdote does not tell whether this led to a statistically significant confirmation of Bohr’ s hypothesis. Note that, for the good guy to actually win the gun duel, the reactive advantage has to be rather substantial. The good guy will only initiate his action after he has processed the visual input that tells him that the bad guy has acted. This will give the villain a serious head start. But there are more reasons to suspect that Bohr might be wrong. For both self-initiated and reactive movements, the action is carried out after the pre-motor planning has finished, since both parties have anticipated the action to be performed. Since all preparation for the action is over, why would it matter that the action was initiated or reactive? Furthermore, in the initiated case, a person will presumably only execute the motion if he feels totally ready Y. Pinto : J. M. Wolfe : T. S. Horowitz Brigham and Women’ s Hospital, Boston, MA, USA Y. Pinto : J. M. Wolfe : T. S. Horowitz Harvard Medical School, Boston, MA 02115, USA M. Otten : M. A. Cohen Harvard University, Cambridge, MA, USA Y. Pinto (*) Roetersstraat 15, Building A, Room 6.05, 1018 WB, Amsterdam, The Netherlands e-mail: yair.pinto@gmail.com Atten Percept Psychophys (2011) 73:613–620 DOI 10.3758/s13414-010-0057-7