And I thought the US move to define the poverty line as the 40th percentile of income (or something lower; I don't remember this too well) was absurd. The average household income is, by necessity, well above the 50th percentile everywhere. There is no one seriously advocating that over 50% of every country's population is living in poverty.
the poverty line as the 40th percentile
of income [...] was absurd.
In the past (19th century and before), the socialist/communist tradition had a real point in that large swathes of the population were poor in an absolute sense (e.g. going hungry, cold, no adequate health-care, pension etc). The socialist/communist tradition was in some sense about abolishing absolute poverty (whether the proposed methods were helpful or not is another discussion). If you read e.g. the communist manifesto [1], that is clear. Most of socialist/communist politics centered around politicising and radicalising the absolute poor with a view towards taking them out of absolute poverty.
However, modern industrialised society has (simplifying a bit) abolished absolute poverty.
This leaves socialist/communist politics in a conundrum: how to continue to politicise/radicalise people to become followers. The concept of relative poverty is the answer: all the old political tools can be reused by changing what is meant by poverty.
[1] https://www.marxists.org/archive/marx/works/1848/communist-m...
Second "when some people get a lot richer" this especially pushes up the cost of housing (arguably education too) a rationed good. The experience here in the UK is that more and more people are renting from those who have the money to corner the housing market. A vicious cycle of money being redistributed upwards.
No doubt type 1 and type 2 poverty are linked.
Over what time period do you mean? I'm pretty sure just about everything has a lower cost than it did 50 years ago, with the exception of real estate in neighborhoods where the population density has soared. If you don't insist on living in one of those neighborhoods, you can life that is better than what you'd have had in 1965 in every respect and spend less time working to support that lifestyle than you would have spent at that time for the inferior quality of life.
But, I suppose it begs a question of why we discuss poverty, at all? I presume it's because some of us don't like to see suffering or despair. Or, we don't like some people being left out of prosperity. Or, are there other reasons?
1. People are poor if they can't afford (basic) things. People are very poor if they go hungry to bed.
2. People are poor if they earn less [1] than the average (not median) household. From the article "The report looked at the gap between the income of the poorest Japanese families with children and the average household income."
When does the poverty rate rise in 1. and 2.?
1. When peoples (real) income decreases (e.g. they lose jobs) and they can't afford (basic) things they could afford 10 years before. Or prices are rising to the same effect. The margin between income and costs gets smaller.
2. When some people get a lot richer (e.g. the "1%") then the income average rises and the poverty rate rises, although "poor" people might also have higher income or can afford more things than 10 years before (because of cheaper food, clothes, phone costs, energy they have more "disposable income").
I wish we would label these two types of poverty differently to have better discussions.
[1] EDIT: a certain percentage