One of the first signs of overpopulation is unemployment i.e. too many people and not enough stuff for them to do.
Third cause; This correlation does not imply causation.
This is only a valid symptom within certain preassumed conditions, and in other contexts fails horribly as a metric.
For illustration, a certain fictional island could have rather poor and difficult source of food (farmland, fish populations, etc.) that simply cannot sustain a population past X, regardless of whether the remaining (N-X) humans have work to do (infrastructure, repair, killing the Giant Death Crabs that prey on the farmers, etc.). Similarly, its sister island could have a large population of wild fruit-bearing trees that can freely sustain any number of humans up to Y, whether or not some of them find themselves without "work" or any skills valuable to other inhabitants.
Both of the above parables are related to real, but complex, issues with natural resource distribution and acquisition in the real world.
Overpopulation, by contrast, is when the net resources of a timespace / group / system can no longer reliably support the population by whatever metrics are necessary or employed by the population -- food, clothing, shelter, kinship, entertainment, and yes, even work, employment and unemployment.
But all of these depend on the context. If you fix the problem of humans requiring food, then food no longer needs to be considered as a metric for overpopulation. If you fix the problem of humans requiring employment to gain access to the food (which retains its own separate overpopulation threshold, generally held to be higher than employment in some places), then employment no longer needs to be considered as a metric for overpopulation.
I broadly agree with your point vis. the problem of requiring employment to gain access to food, but the real picture seems to be more complicated. In game-theoretic terms, once supply outpaces demand, demand often expands to fill available supply. If we can afford to provide food (or, more generally, energy and a small amount of material resources) to everyone simply by virtue of their existing, and we don't provide any controls over the level of population, it's only natural for population to balloon to fill up supply so that we again arrive at an equili...
This topic is in vogue, so here's my pitch.
My fellow humans, I have some bad news and some good news. The bad news is that you are likely to eventually enter an enfeebled state, during which you will not be able to independently provide for yourself. Even worse, you will at some point altogether cease to function and then you can no longer contribute to the things you care about. The good news is that both of those problems can be ameliorated by the same scheme – the creation of new humans. The new humans can provide us with the assistance we need as our own abilities diminish. And when we cease to function, the new humans can carry on with the projects we value.
Now, the thing is, creating fully functioning new humans is a huge project, consuming many man-years of work. A person engaged in preparing and outfitting a new human will need to sacrifice a lot of time that could otherwise be devoted to personal leisure and other projects. We currently have a volunteer system for replenishing the population and in many ways this works well. Not everyone is well-placed for creating humans while some people are in a good position to create many. But this system is not perfect and it can be exploited. There are some freeloaders who do not create humans even though they are in a suitable position to do so. Those same people almost always value receiving care in old age and value humanity having a future. But they are relying on the rest of us to provide enough new humans for this to happen while they can devote all their time to other projects and zero time to diapers with poop in them.
Sometimes the non-child-creators justify their decision by suggesting that the projects they are working on are especially socially valuable and thus they can spend time on them in preference to child-creation without violating their duty to society. While it is *possible* that this argument goes through in some cases, it seems suspiciously self-serving. What is especially worth taking into account is that if the humans in question really are so highly valuable, they would statistically have highly valuable offspring. Thus, it seems doubtful in the general case that high-value people refraining from procreating is a net gain for society.
[Poorly conceived section on my personal experiences removed.]