Decision-making capacity

In many Western jurisdictions, the law presumes that adult persons, and sometimes children that meet certain criteria, are capable of making their own health care decisions; for example, consenting to a particular medical treatment, or consenting to participate in a research trial. But what exactly does it mean to say that a subject has or lacks the requisite capacity to decide? This last question has to do with what is commonly called “decisional capacity,” a central concept in health care law and ethics, and increasingly an independent topic of philosophical inquiry.