1 Introduction In Association Football (soccer), assistant referees (ARs) often make mistakes when judging offside offences. An attacking player should be penalized for being in an off- side position if, at the moment the ball touches or is played by one of his or her team (usually, the instant the ball is passed to him or her), he or she is closer to the opponents' goal line than both the ball and the second-to-last defender (see figure 1a). ARs sometimes raise the flag to indicate an offside offence when the attacker was actually in an onside position at the time of the pass, a situation Oudejans et al (2000) called a flag error (FE), and sometimes they do not raise the flag when the attacker was in fact offside at the time of the pass (a non-flag error , or NFE). An explanation for FEs might be that the errors arise from the time delay taken by the AR to shift gaze from attacking passer to attacking receiver. This delay could be long enough for the receiver to be perceived as offside when, at the time the ball was passed, he or she was actually onside. By equipping ARs with a head-mounted camera, Oudejans and colleagues (2000) disproved this simple hypothesis, showing that errors were not accompanied by shifts of gaze. Oudejans et al (2000) proposed that both FEs and NFEs are probably caused by a geometrical/optical effect, which inevitably leads to errors by ARs. This geometrical effect would arise from the viewing angle adopted by the ARs, who were observed to position themselves, on average, a little over 1 m ahead of the offside line (Oudejans et al 2000), as shown in figure 2. In a much-debated phenomenon, the flash-lag effect (Nijhawan 1994; Baldo and Klein 1995; Purushothaman et al 1998; Whitney and Murakami 1998; Brenner and Smeets 2000; Eagleman and Sejnowski 2000; Krekelberg and Lappe 2001; Nijhawan 2001; Baldo et al 2002), a moving object is perceived as spatially leading its real position at an instant defined by a time marker (usually a briefly flashed stimulus). Here we propose that the flash-lag effect is a likely nominee for a supporting role in explaining the ARs' errors. Figure 2a shows how the geometrical/optical effect reported by Oudejans et al (2000) leads to a bias in favor of FEs when the attacker goes left (left trajectories), and to a bias in favor of NFEs when the attacker goes right (right trajectories). In many FE and NFE situations, the attacker that receives the ball is running towards the opposing goal at the time the ball is passed to him or her. In this case, the receiving attacker is Flag errors in soccer games: the flash-lag effect brought to real life Perception, 2002, volume 31, pages 1205 ^ 1210 Marcus Vinicius C Baldo, Ronald D Ranvaud, Edgard Morya Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of Sa¬ o Paulo, Sa¬ o Paulo (SP) 05508-900, Brazil; e-mail: baldo@fisio.icb.usp.br Received 16 November 2001, in revised form 15 March 2002; published online 18 September 2002 Abstract. In soccer games, an attacking player is said to be in an offside position if he or she is closer to the opponents' goal line than both the ball and the second-to-last defender. It is an offence for the attacker to be in an offside position and in active play at the moment a fellow team member plays the ball. Assistant referees often make mistakes when judging an offside offence, probably because of optical errors arising from the viewing angle adopted by them (Oudejans, Verheijen, Bakker, Gerrits, Steinbru« ckner, Beek, 2000 Nature 404 33). Looking more closely at Oudejans et al's data, we show evidence that the flash-lag effect may contribute significantly to these mistakes. Participation of the flash-lag effect in assistant referees' misjudg- ments would take this perceptual phenomenon from laboratory setups to a real-life situation for the first time. DOI:10.1068/p3422