I conduct two experiments to examine the effects of different types of stress on individuals' willingness to cooperate. The experience of stress is characterized by the primary cognitive appraisal of threat. It activates the emotion of anxiety and induces stress coping behaviors. I posit that because different types of stress differ in terms of the secondary dimension of cognitive appraisal, the responsibility of possible failure, they lead to different stress coping behaviors in collaborative contexts. Based on the attribution of threat, I classify stress into two types, ego and external stress. Under ego stress, the possible failure is attributed to one's capabilities. Ego stressors, such as lack of skill, cause individuals to worry about their capabilities, posing a threat to goal achievement. I argue that ego stress motivates an individual to seek affiliations for joint protection. I provide experimental evidence that ego stress increases cooperation. Under external stress, on the other hand, the possible failure is attributed to factors in the environment. External stressors, such as environmental uncertainty, cause individuals to worry about threat related factors in the environment, which also may hinder goal achievement. I argue that external stress motivates people to avoid risks, including the risk of being exploited by a partner. I provide evidence that external stress reduces cooperation.