As far as I know, Altered Traits is the current best lay summary of meditation research. Some highlights:
There is a TON of meditation research out there, but the vast majority has serious methodological flaws. For example: not distinguishing between types of meditation, not keeping track of lifetime hours of experience, not using active control groups, etc. Still, there are a few findings that seem solid.
One is that meditation follows a dose-response relationship that doesn’t seem to plateau even at the highest levels of experience. There are significant differences between people with 10,000 lifetime hours and people with 20,000.
Another is that mindfulness-based stress reduction (MBSR), the 8-week program developed by Jon Kabat-Zinn (probably the most widespread form of meditation in the West) has real health benefits, and it especially helps people reappraise pain or illness. That being said, some of MBSR’s health benefits appear to come not from the mindfulness training itself, but from the greater focus on healthy living, usually with a community of others who are trying to make similar changes. Also, those benefits fade without continued practice.
Another is that different types of meditation have different effects, not only on one’s states (in-the-moment experiences), but also on one’s traits (long-term, stable characteristics). Mindfulness and vipassina train focus, and lessen reactivity, whereas loving-kindness trains compassion.
Loving-kindness meditation is pretty interesting. It’s much less discussed than mindfulness, but it appears to have a much faster and more noticeable effect. Just a few minutes of loving-kindness meditation can boost a person’s mood in a lasting way. It also seems to increase a person’s likelihood of acting to help others who are in pain, possibly through a dampening of emotional empathy (à la Paul Bloom’s Against Empathy).
The craziest thing I learned in Altered Traits was that there are yogis out there who literally meditate all day every day for their entire lives, and it leaves a noticeable footprint on their brains. These people’s brains have near-constant gamma waves, which are the highest frequency neural oscillation in humans. It’s unclear to me how gamma waves are related to experience, but the authors suggested that you get a burst of gamma waves when you, say, bite into an apple.
After reading Altered Traits, my overall impression is that meditation research (or “contemplative science”, as it’s now sometimes called) is still pretty young. I’ll be interested to see how it develops over the years. I definitely recommend the book, if the topic interests you.