Information about environmental dangers is valuable and the cost of transmitting such information to social partners is minimal. Furthermore, an information transmitter who helps a social partner to avoid harm can later reap the benefits of a continued social relationship, and does not have to pay the costs of helping an injured friend. This cost-benefit assymetry suggests there is a positive selective pressure on a propensity to socially transmit information about danger at especially high rates compared to information that is not about danger. We call this predicted propensity a warning bias. Here we report the results of tests for this bias using data from the social networking site Twitter. Two coders rated each of 13,203 tweets (publicly-shared 140 character utterances) for whether or not each tweet contained information about danger. The number of retweets for each tweet indexes that tweet's transmission rate, and was our outcome variable. Results of negative binomial regressions showed that tweets about danger have up to 3.13 times as many retweets as tweets that are not about danger.
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