Companion dogs are widespread in western countries, and scientific studies have proved that dogs have a number of positive effects on human health and well-being. Studies have shown reduced systolic blood pressure in dog owners compared to non-owners, as well as lowered concentrations of plasma triglycerides and cholesterol. Studies have also shown improved survival rates following myocardial infarction in dog owners compared to non-owners. Companion dogs are used systematically in "animal assisted therapy" in various institutions and hospitals, both as specific treatment of a medical condition and to improve well-being in certain groups of patients. The reasons for the positive effects of dogs on human health are not clearly identified. The attachment between people and their pets ("the human-animal bond") seems to have important physiological and psychological effects. Companion dogs have been shown to increase physical activity and social contact, which may also influence human health.