I've got a PhD in a specific subfield of psychology (social, clinical, developmental, etc.). I happen to be a professor at a small college, but am building a side business into to a full-time business. Over the last 5 years I've made money from consulting with businesses who need my expertise (staff training, Continuing Education Presentations, product development, writing content for newsletters/trade journals, program development (stress reduction program for a specific profession), speaking (state and national conventions), product development (printed materials; some sold to individuals, some sold in bulk to businesses that want to provide it to their clients), website and social media content.
What I've learned is that in any business/field, there is a huge amount of trade journals (I'm not talking about academic/research journals), newsletters (professional organizations, large businesses, etc.) that constantly need content. Sometimes you can get paid to write this content; at minimum you get name/brand recognition. I don't know your field, but here are my guesses for some possible ideas for income streams:
- become a consultant/advisor to a national team, professional team, high-end sports training facility, private school, etc.
- write books, booklets, training manuals/protocols, blog content, website/newsletter content, etc.
- become known for your area (make it big enough for some side projects/topics, but small enough that you are known as one of the top 5 people in the topic)
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This has been my formula for making a living with a PhD outside of academia (it ain't perfect and I'm still learning):
- find a niche (something smaller than your subfield - expert at a specific sport, expert at re-entering sports after a specific injury, etc.); know your shit
- brainstorm individuals would use your information/expertise
- brainstorm businesses/organizations who also work with those people
- sometimes it can be much easier to look for companies who want to work with those individuals and then help the company provide better service to those individuals. The company sees it as a way to differentiate themselves from their competition ("Use us, we have a Phd on staff/ consultant who will REALLY help you!")
- develop some products/services on your own. 1 hour presentation, 2 hour workshop, training protocol, booklet (8-40 pages), book (although the time investment and potential return are iffy), etc. Find someone who needs this information. Start small and cheap - get feedback on your product/service. Refine it. Expect that a completely different product or service is what they really need and be willing to jump to that. For example, I started with printed materials (booklets) that I thought would be really helpful. But what businesses really want is someone to have a column/feature in a monthly/quarterly newsletter/magazine - ok, guess I'm an article writer instead.
- go to places where these businesses/individuals interact: conferences, conventions, training sessions, etc. Listen for problems people and businesses are having - propose solutions. Get to know the people in the field - having a degree will help get people to listen to you, but having a degree + a relationship is how you get people to buy from you. Get to know them and learn from them. Listen. listen. listen.
One significant problem many PhDs have when starting is undervaluing their expertise and degree. I'll be the first to say that many people give PhDs too much credit- in some ways. I have a company right now that wants me to create a program for their employees on stress reduction. I think they assume that I've had several classes that focus entirely on stress reduction. As a psychologist, I know about stress-reduction, but it isn't something I specialize in. But I do have the background to learn and create a program and recognize good research from crappy stuff. In the academic world, we often specialize so narrowly that you won't dare speak up against someone who has been studying the concept longer or written a lot about it. In the non-academic world, they are much less concerned with the perfect research-based answer and simply a reasonable answer that provides value. In short, you'll likely feel like an imposter ("I don't have the specific expertise and years of research to do that!"), but the real answer is that you have learned skills (general knowledge on the topic, ability to find, evaluate, & integrate information, etc.) to create things that haven't been developed or haven't been adapted for a specific situation.
....and now reading back through your question, I realize that you may be asking if getting my PhD was a tough process. It was a lot of work and I don't want to discount how much time I put into studying, writing, etc. But the reality is that getting accepted to a PhD process is the tough part (at least in my field which has acceptance rates at or below med school acceptance rates). After they admit you, they want you to succeed. It makes them look bad if they have to report to their accrediting agency or university that they are accepting a bunch of students and only 50% finish - this makes the professors look bad. So they will push you to do good work, but they would hate to see you drop out. That's my experience. YMMV.