If you know nothing about them you might want to look for whether they’re emotionally expressive. People who showed more emotion – positive or negative – were more cooperative in research studies:
Previous research has suggested that the spontaneous display of positive emotion may be a reliable signal of cooperative tendency in humans. Consistent with this proposition, several studies have found that self-reported cooperators indeed display higher levels of positive emotions than non-cooperators. In this study, we defined cooperators and non-cooperators in terms of their behavior as the proposer in an ultimatum game, and video-taped their facial expressions as they faced unfair offers as a responder. A detailed analysis of the facial expressions displayed by participants revealed that cooperators displayed greater amounts of emotional expressions, not limited to positive emotional expression, when responding to unfair offers in the ultimatum game. These results suggest that cooperators may be more emotionally expressive than non-cooperators. We speculate that emotional expressivity can be a more reliable signal of cooperativeness than the display of positive emotion alone.
Source: “Emotional expressivity as a signal of cooperation” from Evolution & Human Behavior, Volume 31, Issue 2 , Pages 87-94, March 2010
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