I think relevant information of very good quality, meaning not just any information but information with a high signal-to-noise ratio for each person in each moment in time. Or maybe any tool that really helps hiding the information which is not of very high quality, relevance and level for you in any particular moment. I think information overload is the main externality of the information age.

At the end, there are only few main resources: energy (this includes food and water) and (good)knowledge-time-attention. I think knowledge, time and attention form a bundle of resources that are very related but difficult to summarise within a wider resource.

I find the concept regression toward the mean very useful because it is very intuitive, understandable and not related to health. Especially this example of Kahneman about an instructor of the Israeli Air Force is totally relatable, funny and understandable.

The fact that this concept (and the example) is not related to health has the advantages that one doesn't need to have any kind of health-related knowledge to understand it and that it may not trigger usual arguments. The disadvantage is that they may argue that it does not apply to health. But there is a workaround: start by pointing out that sometimes one does feel ill and this feeling goes naturally away --the body is a healing machine! They will have to agree on that. Then you can tell the story.

I tried something similar a couple of times online with total strangers... I'm really not sure of the outcome. But face to face it may work better, especially with family members.

Good luck!

[Disclaimer: although I felt very bad at the moment, I have never sough mental medical advise/treatment (something which I think is quite common). Therefore, I was never diagnosed of depression.]

Relocate. I know it is much easier said than done, but sometimes the chance just arises at the right moment. I guess that in a large community the chances for someone to be able to do it are quite high.

I left home, country, family and friends for 10 months for an study exchange (Erasmus) and it was a game changer. If you are lucky and with a bit of work, you even get to "choose" the people you interact with --and that most likely after some time will become friends-- probably making you feel more connected to your friends in the future.

Since then I've been abroad for more than 10 years. I didn't come back home for any period longer than ~6 months and sometimes being back has been challenging. This is because it is easy to slide back to old routines and behaviours and, therefore, feelings. However, and this is very important, when I did mentally prepare to avoid sliding back, it has been much better. By mentally preparing I mean just to think on the usual issues you had, and plan how to avoid, prevent or manage them. I then try to recall those thoughts from time to time when I'm actually back.

Having something that makes the experience of being back very different also works for me. Whenever my girlfriend has come (including the ~6 months period) all dynamics have naturally changed and I have felt very well (even though I think every time she's came I have forgotten to mentally prepare before the visit). 

I don't think distance is what is significant but the fact that you are in an unknown and new environment. However, distance helps to make it difficult not to come back for a significant period of time, which helps you develop and settle in a new and happier mental state. I suspect that the fact of being in a country where a different language is spoken may also help because the environmental change is dramatically more significant.

I ignore if this is scientifically tested in any way. I do recall it mentioned in this 80000h podcast. So my advise would be not to plan a relocation only as a therapy against depression --although it doesn't seem uncommon for people to relocate just to escape their environment; indeed it played a significant role in my decision. Rather, if you have the opportunity, the curiosity or just the will, I encourage you to try it and I think this will contribute significantly to improve your well-being.

I would add a general comment here: a relocation is also a good chance to change something one does not like of himself, e.g. stop smoking, change a particular behaviour. Often these behaviours are (at least partly) triggered by environmental clues that are altered when you move, making it much easier (this does not make it easy, though!) to modify them... If you have previously mentally prepared!

I hope it helps.

I don't know how well the studies on which is based replicate, but 'Predictably irrational' from Dan Ariely is also in the Behavioural Economics field, and Dan seems to put a lot of effort into preparing valid experiments.

I think you are pointing in the right direction when you say you understand that societies that discourage murder probably have better prospects than those that do. I'm far from well read in this topic but I think the basis of ethics/morality (in a general way, at least) is our evolution.

A collective with a feeling that murdering is bad will, in the long run, outlive a collective without that feeling. Keep that feeling for long enough and their society as a whole will rationalise it. Something similar can be said for, at least, many of our other moral preferences.

Societies which embed these "feelings" better in their norms will tend to outlive the other societies and, at the same time, will reinforce those feelings. In ancient times before proper justice institutions that could enforce these norms, religions were a good way to guard them. 

Thinking this way, morality is broadly the set of feelings we have that help a society (or pack, or family, etc) to thrive. We have them because we are descendents of the members of the societies with 1) more subjects having roughly these feelings and 2) which were better at reinforcing them (the feelings) culturally.

As side note, I am pretty convinced that incest and inter-family romantic relations will stop being morally wrong in the future (in a similar way that homosexual relations are not morally wrong any more). Incest being morally wrong makes sense when the purpose of having sex is reproduction, because it may lead to progeny with problems. However, sex and progeny are now pretty much decoupled. I don't see any reason why, if two members of the same family want to have a relation, they should not. And, indeed, I don't feel any moral reaction to that. It may take some time, as it is taking quite some time for humans to develop feelings for humanity as a whole (as opposed to in/out group thinking), but I think the process is running already.