Regarding the initial questions @Noone-is-here raised in this thread - I thought of adding the following ‘final thoughts’:
In Willoughby’s research (and in the other articles that were listed by Noone) - the research focus is on (i) quantifying the number of people that are ‘adversely’ affected by practising meditation, and (ii) listing the different ways in which people are affected (re-living trauma, having worse moods, emotions, etc.). In this research, these ‘adverse effects’ are talked about as if they are ‘injuries’ that have unfortunately happened to people as a result of meditation!
However, these researchers conduct their investigations by making a huge incorrect assumption. They assume that meditation is about ‘inducing’ something into the body - therefore they think that just like how drugs have various effects if they are taken in excess, meditation practices can have unintended “adverse effects,” and therefore ‘too much meditation’ can be bad. I have also heard Willoughby comparing too much meditation to drinking too much coffee, and she talks about what ‘dose’ of meditation is ideal for people, etc.!
Now, if we look at the teachings of the Buddha, we see that meditation and mindfulness practices are carried out to understand reality (i.e., to understand the true nature of human experience). In the Four Foundations of Mindfulness, the Buddha describes how different practices contribute towards these understandings, and also explains ‘causality.’ How meditation can be cultivated in all four postures is also explained by the Buddha and this means mindfulness meditation can be done all the time one is awake. There is no dose restriction! This alertness (mindfulness or ‘Yoniso manasikāra) is what gradually leads one to wisdom and insight.
So, if any of the so-called ‘adverse effects’ happen within meditation, these experiences can be insightfully seen within the Buddha’s guidance, especially in terms of overcoming the Five Hindrances. The five hindrances are: Desire, Ill-will, Sloth/torpor, Restlessness/worry, and Doubt. Now, if we contemplate, it is possible to see that ALL so-called ‘adverse effects’ fall under these five hindrances. For example, ‘reliving past trauma,’ ‘worse mood,’ etc., are about hate/ill-will (i.e., hating the people involved in that past event/thought, hating to experience this emotion, etc.). Also, if one really ‘wants’ (i.e., attaches to the idea of) a peaceful meditation - this is about the hindrance of desire. Restlessness/worry can also come up even as a result of thinking whether one might experience ‘adverse effects’ from meditation! (the way to overcome this is to observe this thought as it is - coming and going). So, if all experiences are viewed as present moment experiences, we cannot talk about ‘adverse effects’ and therefore they would not negatively influence a person - instead the practices gradually take the person to a peaceful happiness and even contentment regarding ‘seeing things as they are.’
Therefore as I see it, if researchers really want to study the ‘adverse effects’ of meditation, then it would be more helpful to study how a person can overcome these effects and continue to progress in meditation (instead of telling people to not meditate or warning people about these ‘dangers’ - unfortunately this is what is happening now).