Although psychologists have urged police officers to use double blind line-up procedures during their investigations, police officers state that these would be difficult to administer and most have been reluctant to implement this change. Four studies examine whether lay people's judgements about the guilt of a suspect vary according to whether a brief written summary of a case described the identification procedure as double blind or non-double blind. The effects were all small (and almost all non-significant). Most people do not treat double blind line-ups differently from non-double blind line-ups when assessing the guilt of a defendant. Either police investigators should stop using this biased method or police investigators and others in the judicial system (e.g. jurors, judges) should be informed of this bias when evaluating results from any line-up. Copyright © 2009 John Wiley & Sons, Ltd.