Effectus spectatorius est res psycho-socialis, quae ad talia refertur eventa, in quibus nemo ex spectatoribus qui adsunt victimae auxilium fert. Quo plures numero spectatores, eo minus veri simile est aliquem ex spectatoribus ad auxlium venturum esse.

Effectus spectatorius disputari coeptus est, postquam Kitty Genovese, femina duodetriginta annorum, Novi Eboraci coram in via stuprata ac cultello occisa est anno 1964, nec quisquam ex paene quadraginta spectatoribus ipse victimae moribundae ad auxilium venit. Quo homicidio et pigritia quadam eorum qui aderant obstupefacti psychologi sociales ceterique homines, quare eiusmodi facinus sine interventione spectatorum committi potuerit, admodum mirabantur. Ex clarissimis libris has quaestiones tractantibus est The Unresponsive Bystander: Why Doesn't He Help?, quo Bibb Latanë et Johannes M. Darley auctores experimentis causas effectus spectatorii cognoscere temptaverunt.^[1] Multis enim factis experimentis cognoverunt interveniendi voluntatem eo maiorem esse, quo pauciores spectatores sunt. Et contra, quo plures spectatores, eo minor interveniendi voluntas. Quam pigritiam socialem fieri putant ex dispersione responsalitatis: ceterorum esse victimam adiuvare. At ubi primum homines de re satis periculosa agi vident, attenuatur effectus spectatorius.^[2]

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• Kassin, Saul; Fein, Steven; Rose, Hazel (2010) Social Psychology (410-2)
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• Latané, B. & Darley, J. M. (1970) The Unresponsive Bystander: Why Doesn't He Help? Appleton-Century Crofts
• Manning, R.; Levine, M.; Collins, A. (2007). The Kitty Genovese murder and the social psychology of helping: The parable of the 38 witnesses. American Psychologist 62: 555-562.
• Stalder, Daniel (2008) Revisiting the issue of safety in numbers: The likelihood of receiving help from a group. Social Influence 3: 24-33.