Behavioural change is the first notable symptom. It's the type of behavioural change that pin points which demon has got into a person.
Of the active underworld entities in Ireland the ''Trow'' and it's elk are the most common and most dangerous. Their quest to co exist with humans involves possession and then mating with/through humans to produce an off spring known as changlings.
From the moment of possession the host becomes a changling and so any future children become the same were disease and deformity occurs along with a trend of misfortune and accidents around the Quarter days.
These entities are not of our World and so the disease factor strikes. Lives of wreck and ruin are the consequence.
When you speak of ''the extent of our experiences'' as a factor in developing mental conditions and so on you have to allow room for demonic type possession as a contributing cause.
If you've never had such an encounter there's little i can say or do to convince you otherwise although you were invited to the grove to see for yourself remember?
I'd have jumped at the chance. Still, if there's anyone out there with all that ghost busting equipment or whatever, pm me and we'll apply the science to the place.