Questions
Who do you ask the questions? Most of us are surrounded by people who are very much like ourselves.
I was lucky, in a way, to grow up poor, and to have spent years in the company of backwoods Appalachian people, to have the run of seedy bars, etc. But I've also, by choice, spent a large part of my life going to some new place each weekend, even if it's just a local political dinner thrown by a political party opposite of my own, or a chili supper held by the local PTA, or a square dance, or a mountain man festival, or a choke and puke restaurant. And everywhere I go I talk to people. More important, I let people talk to me while I listen. Just about everyone loves to talk about themselves and the problems they face. The person willing to listen is a rare commodity.
There are many ways to do such research, including a lot of reading, but being surrounded by people much like ourselves is always a choice. I've found that most of the best writers out there find ways of spending a lot of time with people from every class and division of life.
The ability and willingness to spend time outside of your comfort zone and regular assortment of friends and colleagues is invaluable.
Newspapers and libraries and bulletin boards everywhere have upcoming events on them.
But I would say there is no such person as a single, illiterate mother who has never heard of birth control. Every person in the US, and I'd say 99.99% of the people on the planet, has heard about at least two forms of birth control, no matter how backwater they are, and no matter how illiterate they are.
It isn't ignorance of birth control methods that makes a women get pregnant, it's not using the methods they know about, and there are many, many reasons for choosing not to use birth control, ranging from the fact that the condom is still the main choice for many, and large numbers of men hate using condoms, to the belief that you can't get pregnant the first time you have sex, to being drunk, to actually wanting to have a baby.
Being middle class in no way means you can't spend a good amount of time with people of the lowest classes. This is easy, and takes little effort.
Something much more difficult, infinitely more difficult, is finding ways to spend time with the super rich.