It makes sense that your subject will have an influence on how much of a return your site brings in from Adsense. After all, the number of readers will have an impact on the number of people checking out the ads, and the topic will inevitably have an effect on the number of readers. As much as you may love French jazz music, for example, it’s less likely to pull in big numbers than Hollywood blockbusters.
This leads many people to think that they should ignore the niches that are more esoteric, which in fact really define the term “niche“. However, there is a growing body of opinion that suggests that, contrary to the received wisdom, a beginner should start with a lower-paying niche. Sure, it won’t bring in big money, but it is a gentler learning curve.
The bigger a crowd gets, the narrower its interests are. One person may like French jazz, as mentioned above. Then if you put them in a room with 99 other people they may be the only one who likes French jazz, one of ten who like jazz, and one of fifty who like music. It goes without saying that the more specific you get, the less interest there will be.
To reach a bigger crowd, you need to be more general, but the tricky part is that there are already plenty of sites catering to that. What you should look to do is begin by writing about what you know, so that you learn the process without having to stick to something you are less sure about. As you learn the vagaries of the process, you can then build a money-maker using what you have learned.