Claim: A frown is used to express negative emotions.
Evidence: While the appearance of a frown varies from culture to culture, there appears to be some degree of universality to the recognition of the frown as a negative facial expression. This universality suggests a shared adaptive quality to frowning allowing for social communication of negative emotional states. In fact, frowning as a component of anger or disgust is widely recognized as a universal expression easily recognized across cultures. 
In social settings frowns are most frequently used to express a range of negative emotions including anger and displeasure.