Today I've been told by the vet to consider spaying our almost 3-year-old Jindo dog. We haven't spayed the dog for four reasons:

1. A while back, doing some light research on google (meaning I only read the first few search results) revealed that spaying brings on its own set of problems, and haven't found any article, particularly scholarly ones, that pushes the spaying from "recommended" to "you must."
2. I just find it hard to that the product of evolution has resulted in a species with so many health issues. While a lot of modern dogs, specifically those from "developed" nations have been bred for particular traits over generations, sometimes resulting in overbreeding that results in over-elongated dachshunds or snore-y pugs, Jindo dogs, from my understanding, haven't gone through such human intervention and simply followed the "survival of the fittest" rule.
3. My dog, without much effort on my part, has been healthy: she eats well, walks well, runs well, inhibits healthy squirrel chasing dog behaviors, and stays trim all on her own. At the very minimum, I walk her 40 mins a day to up to an hour every day. So far, she's seen three different vets in three different places we have lived, and they all said she is healthy. Combined with reason #2, I find it hard to believe that such a healthy dog would just start to develop problems because such problems are inherent to the species.
4. Lastly, I don't want to alter her in any way unless necessary. It's a medical procedure, and we're messing with biology. More than anything, it's an ethical issue; my dog may not be able to talk, but she has a right to her own body. Barring any medical emergency, I want to respect that right.

I hope I don't come across as some fringe anti-vaxx, flat-earther kind of person by expressing my skepticism about the generally accepted practice of spaying and neutering. My view is partly tinged by the fact that I know vets and animal shelters recommend it for the reason of controlling the animal population. (my dog is leashed 100% of the time she's outside, except on rare occasions when we go to the dog park, which we don't go unless we're 100% convinced that she's not in her heat cycle). 

All that is to ask: where can I find some definite, credible statistics on the health effects of spaying dogs? I know it's hard to find articles on such a generic topic, so if I can get some sources on the effect of spaying on breast cancer, or some other major life-threatening issues in intact dogs, I'd really appreciate it. 

According to this article (<https://onlinelibrary.wiley.com/doi/full/10.1111/j.1748-5827.2011.01220.x>, in the conclusion), this issue apparently isn't as studied as much as the rate of recommendation would have you believe.