Loving-kindness practice has become the subject of an extensive contemporary psychological research program, and describing a few findings shows how a classical technique has been operationalized and tested by contemporary science.
Barbara Fredrickson, working within her "broaden-and-build" theory of positive emotion, led randomized trials in the 2000s finding that a several-week course of loving-kindness meditation produced measurable increases in participants' daily positive emotions, which predicted improvements in personal resources, including stronger social connections and greater life satisfaction, giving researchers a testable mechanism for effects the classical tradition attributes to the cultivation of metta itself. Other researchers have used brain imaging to compare experienced meditators with novices, finding differences in regions associated with empathy, and clinical trials have tested loving-kindness meditation for chronic pain, social anxiety and post-traumatic stress, with generally positive results.
This research illustrates a broader pattern: specific practices, extracted from the fourfold scheme and its near-enemy and far-enemy analysis, are tested using methods largely indifferent to the doctrinal context, rebirth, karma, the bodhisattva path, that gave the practice its original meaning, a pattern some welcome as evidence the practices work and others criticize as extracting a technique while discarding the worldview that gave it its point.