Benefits of Loving Kindness Meditation
Loving-kindness meditation is a special type of meditation meant to engender feelings of unconditional benevolence towards all sentient beings. One of the most commonly practiced ways to go about it is to start with easy subjects to project feelings of love towards, such as a loved one, yourself, and a friend then move towards neutral persons such as a stranger and lastly to difficult persons like an enemy or someone that one is having trouble dealing with. Finally, one will project this feeling they have spent this whole time building up to towards all sentient beings in the universe, unconditionally.
If one has never experienced the feeling of what seems like "infinite benevolence," it is quite a pleasurable one. Over time, it is said, the amount of "problematic" persons that are available to be subjects of one's loving kindness practice slowly dwindle away, which tends to increase relationship quality and lower feelings of harsh or negative criticism directed towards others. Or so it is said, at least. Fortunately, research does seem to back these claims up.
Recent Research as of Feb. 14th, 2015
Recently (Published on October 23rd, 2014) Kemper KJ et. al. published a report that assessed the effects of Loving-Kindness on both inexperienced meditators and expert meditators with regards to nitrate levels (related to cardiovascular health) and well-being, measured by stress leve assessment.
The experienced meditators had both a lower level of stress and a lower heart rate (a sign of increased heart health) than the inexperienced meditators. After a brief 20-minute meditation, relaxation increased and stress decreased in the inexperienced meditators. Nitrate levels were more than double in experienced meditators versus inexperienced, which was claimed to potentially explain the cardiovascular benefits of Loving Kindness Meditation.
So, apparently being loving and kind truly does help the heart--both metaphorically and objectively/literally. It seems to also reduce stress levels; a conjecture could be made that this would be a result of reductions in inner tension due to resolved inner conflict related to relationships.
Effects Upon Emotions and Other Variables
In another recent research study published about metta meditation, Kearny DJ et. al. published a paper in December of 2014 that was fairly detailed with regards to various measures, mostly emotional. The study showed that the meditators reported an increase over time in positive emotions, and a decrease in negative emotions. Interestingly, not related to emotions directly, many other important variables increased as well, all of them seemingly salutary. There were increases in purpose in life, environmental mastery(competence during everyday life), self-acceptance, and decentering(the ability to see different aspects of a problem, or, see it "from different angles").
One interesting aspect of this study, is that the follow-up period wherein the participants were checked to see progress was only three months. This would seem to be a good reason to create another study with maybe the same participants--or different ones, with hopefully a more comprehensive study design that checks even more variables--but over a longer course of time. One study proved that after 9 months, an increase in positive affect (displaying positive emotions, e.g. happiness, pleasure) that seemed to be the result of loving-kindness meditation was the same as shortly after the meditation classes had ended... even if they had stopped their practice! If after stopping for over 6 months the benefits gained remain, and are cumulative up to three months, this seems to definitely warrant a longer-term study to at least assess the limits of the benefits, and where they taper off, if at all. Who knows, it may happen!
A free practice that seems to only have benefits that slowly build up over time and persist even after one stops: if that doesn't convince one to begin a practice, not much else probably would.