The most well-known writing on this is the book "The Spirit Level: Why Equality is Good for Everyone". Its authors, Richard Wilkinson and Kate Pickett, have recently released a new book specific to mental health: "The Inner Level: How more equal societies reduce stress, restore sanity and improve everyone's wellbeing".
In both books they outline the relevant research in detail. The central point is that economic inequality and rates of mental illness - including depression and anxiety - are clearly related.
The most unequal countries - such as the UK and the US - have the highest overall rates of mental illness. Meanwhile, the more equal countries - such as Norway, Finland and Japan - have the lowest.
On most of these measures, New Zealand sits somewhere in the middle of the top half of the most unequal.
But the most surprising detail is that everyone, even the most well off (those in the top 10 per cent income bracket), are worse off in the highly unequal countries, when compared to more equal countries.
We often talk about the pressure to "keep up with the Joneses": that natural feeling to compare ourselves with others, to look at those who have more, and feel we need to strive for the same.
But status anxiety is more pronounced in societies where incomes are more unevenly spread.
Even those considered wealthy, by any objective measure, are not immune to the impact of ideas that to keep up the big house, the private schools and their place in the world, hard work is required: and it could be lost at any moment.
Under these conditions anxiety flourishes. And it makes those susceptible to anxiety and depression more likely to develop those symptoms.
Interestingly, it seems unrelated to the actual amount of money you earn, and more about how we perceive ourselves in terms of "social rank". It's human nature to be finely tuned to where we fit, and the impact of that social appraisal is the key to status anxiety.
Of all the people who understand this, advertisers get it most clearly.
They use it to sell us stuff: the car we drive, the phone we use, the clothes we wear, the house we live in.
All these things cost money. But more importantly they tell the world about our social rank, about where we fit. And to sell us a new phone - for instance - is not just to buy a communication device. It is to buy a social status symbol.
Surprisingly, advertisers spend less in more equal countries - such as Denmark - because they are less motivated by this kind of status purchasing.
So while it is human nature to want to keep up with the Joneses - and it's easy to see the harm that it can do - it's becoming increasingly clear in unequal societies that keeping up with the Joneses is also harmful to the Jones family, and indeed to all of us.
Where to get help
Lifeline: ph 0800 543 354 (available 24/7).
Suicide Crisis Helpline: ph 0508 828 865 (0508 TAUTOKO) (available 24/7).
Youth services: ph 06 3555 906.
Youthline: ph 0800 376 633.
Kidsline: 0800 543 754 (available 24/7).
Whatsup: ph 0800 942 8787 (1pm to 11pm).
Depression helpline: 0800 111 757 (available 24/7).
Rainbow Youth: ph 09 376 4155.
Tutaki Youth Inc: 06 928 4517
Supporting Families in Mental Illness: 0800 732 825
The Rural Support Trust: 0800 787 254
The Women's Centre: 06 758 4957
Progress to Health Taranaki: 06 757 5549
If it is an emergency and you feel like you or someone else is at risk, phone 111.
