When thinking about how to accumulate a million dollars or more, there are three key issues that people must consider.
First, a job must be accessible for it to offer a high likelihood of millionaire status. For instance, playing in a professional sports league dramatically increases the odds of earning enough to become a millionaire, but professional sports employ less than 5,000 athletes (among the big four North American leagues). Likewise, virtually every Fortune 500 CEO gets a million-dollar pay package (or better), but there are only 500 of those jobs available.
What's worse, while I do not understate the importance of hard work for athletes, there's an element of natural talent that must be present for this to be an option. Likewise, while being an A-list movie star or musician certainly pays well, it too relies on an all-too-rare combination of talent and luck.
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Accessibility can also refer to the training required and the number of opportunities that exist in a given field. Economics can indeed pay quite well at the upper echelons, but that often requires a degree from one of a very limited list of PhD programs. Likewise, while there are academic disciplines that can pay surprising well (astronomy, for instance), relatively few jobs come available in a given year.
Clearly, a job must pay well if one is to build a seven-figure net worth from it, so salary is a significant factor. It's not all about salary, though. It is also important for a career to have the duration necessary to build the requisite amount of wealth.