Health and care services for patients may improve or harm the wellbeing of their family carers. Formal consideration of these effects (also known as spillovers) in decision-making is advocated, but, to date, little is known about how they occur. This paper presents the first empirical study to determine the mechanisms by which health and care services affect family carers' wellbeing. The study focused on three major health conditions: dementia, stroke, and mental health.