Finding a good source to explain it is very hard ... the lefty material focuses on income inequality and the righty material focuses on how the average income of the middle class is on the rise. You really need to put multiple sources together to get to the truth which nobody really likes.
<I live with someone who teaches/researches sociology ... now, the middle class isn't her specific area of expertise, but much of the research she does touches on how income affects our behaviors and patterns. I'm a techie, but we have great conversations about what she's teaching / researching.>
There are some very basic facts that help set the stage:
Since approx 1990 the middle class has been shrinking.
About 60% of those who left middle class went into the "upper class" income category.
About 40% went into "lower class" income category.
The average salary of those who are considered middle class has gone up.
The average salary of those who are in the upper and lower classes has also gone up.
This is where it gets tricky... if you stop there, looks good right? Well, you have to look into those facts to see why this isn't so rosy.
By most accounts, middle class is defined as those who make 60% to 200% of the average salary. It also depends on how many people are in your household (two adults vs 1 adult, 1 child have the same salary requirement to sit there because it's two people to house, feed, clothe, etc).
Ok, so middle class is really determined by the average salary of the entire working population? So what?
In the US, the top 10% make 45% of the personal income.
People are making more money - those top 10% are earning a larger chunk of the overall income than they did in 1990.
Wait, you've still lost me, this makes no sense.
This is where income inequality comes into play...
The purchasing power of the upper class has gone up.
The purchasing power of the middle class has gone down (same purchasing power, but less people in it).
The purchasing power of the lower class has gone down even further.
Hmmmm.... so?
Well the lower class has grown, but there is less money to go around to each individual family.
But wait, above you said the salaries went up?
Yes people make more money, but when you look at how the cost of living has increased, wages in the lower class have not kept pace.
And technically poverty has gone down, but the number of people on the verge of poverty is going up.
And those who are poor are "more poor" than before.
Even in lower class, there are two designations .. working poor and underclass.
The US outpaces most first world countries in "working poor" .. you guys work, you just get paid diddly squat for your efforts.
AHA!
"The rich are getting richer and the poor are getting poorer."
I would also be negligent if I didn't mention skin color / privilege. Much of the above can also be broken down further. White people are moving up more than non-whites. It's a fact, deal with it. White privilege is real when it comes to income.
The reality is that more people in middle class sit closer to that bottom 60% threshold to be considered middle class than ever before (i.e instead of a nice distribution through the 60-200 range, people are moving up and down with fewer in the middle). That's what I mean by more people are moving down. People in lower class and lower-middle class are having more trouble than ever in purchasing the necessities of life (food, clothing, shelter). In fairness, many of them are blowing too much money on perks like cell phones but are we really that petty? Everyone one wants nice shiny things and cell phones and internet access are becoming as normal as fruit of the loom underwear. We as a society promote it, require it, and look down on people who don't have a nice new cell phone.
Sorry I don't have any great sources for you, I can try and get some if you're really interested. It may not seem like it, but I'm trying to be considerate of both sides ... many people are better off than they were 20 years ago or are doing better than their parents (I'm one of those .. my old man worked 70-80 hours a week as a landscaper when I was really young just to pay rent and put food on the table. He worked his ass off .. but I'm white and going to university was always in my future). It's a problem that can be solved, but it takes the desire to solve it from the people in the middle class and upper class. At the moment, in the US and Canada there is very little desire from that part of the population.
That's why I don't mind paying my fair share of taxes and I have no qualms about requiring those who make good money to pay either. They're not suffering because of an extra $500 in taxes, but those in the lower class are. Besides, almost 100% of the money they're (lower class) spending on cell phones, food and rent are going right back into the economy. How much of the money Bezos earned in the US is going back into the US economy? You can bet it's a lot less than 100%.