A study published in the British Journal of Health Psychology (D. Wells, vol.12, 2007, pp. 145–156) found that dog owners are physically healthier than cat owners. The author of the study was quoted as saying, “It is possible that dogs can directly promote our well being by buffering us from stress. The ownership of a dog can also lead to increases in physical activity and facilitate the development of social contacts, which may enhance physiological and psychological human health in a more indirect manner.” Identify lurking variables in this explanation, and use this quote to explain how lurking variables can be responsible for an association.