Researchers found that people working in a dimly lit room were more likely to cheat about their own performance on a task in order to earn more money than people working in a well-lit room.

Darkness may create a sense of illusory anonymity that disinhibits self-interested and unethical behaviors. It appears to induce a false sense of concealment, leading people to feel that their identities are hidden.

References:
Dim Lighting and Sunglasses Encourage Unethical Behavior in Study. WebMD.
Image source: Ray Ban Original Wayfarer, Wikipedia, Creative Commons Attribution-Share Alike 3.0 Unported license.

Comments from Twitter and Facebook:

@scanman: RT @DrVes: when working in a dimly lit room http://goo.gl/fb/erLEf

@doctorwhitecoat: Clubbing/bars? RT @DrVes People behaved more selfishly/dishonestly when in a dimly lit room or when wearin sunglasses http://goo.gl/fb/erLEf