Ah, socially responsible investing ("SRI" to those in the know). Who wouldn't want to be responsible when plunking money into American businesses? Who wouldn't want to support the good guys and brush off the firms wearing black hats? Well, okay, I know many investors just don't care all that much. They're more interested in finding the best bargains.

The more I think about that -- and I've done so for many years now -- the more logic I see in it. Here's a big reason why: It's hard to find companies that don't do some suboptimal things. There's just a lot of gray area out there, when you're trying to sort the good from the bad. Consider this: The other day, I clicked over to CorpWatch.org, which "investigates and exposes corporate violations of human rights, environmental crimes, fraud and corruption around the world. We work to foster global justice, independent media activism and democratic control over corporations." There I found a link to a page discussing various industries -- and revealing more problems with each of them than I had thought of. For example, here are some of the downsides of food and agriculture:

"The industrial food chain is complex -- and highly profitable for those who control it. In India, ancient traditional grains have been patented by multinationals, while drought- and pest-resistant strains of food crops are engineered in laboratories and planted in massive monocultures worldwide. Harsh pesticides and herbicides have become the rule instead of the exception. GMO [genetically modified organism] soybeans are crowding out the Amazon rainforests; meanwhile, massively subsidized, nutritionally-challenged corn finds its way into almost every aspect of the American diet, especially fast food. The corporations (Monsanto (NYSE:MON), privately held Cargill, Archer Daniels Midland (NYSE:ADM), McDonald's (NYSE:MCD), etc.) behind what we eat exert power in their best interest, rather than in ours. What's good for their bottom line is not necessarily good for our waistlines, our coastlines, our treelines ."

Even technology and telecommunications firms sport downsides: ". Obsolete electronics pile up in developing world landfills, exposing children to toxic metals; and multinationals tinker with the technology of nature to make a tomato that doesn't spoil on a grocery shelf." And the travel and recreation industries, too: "Cruise ship operators are under fire for dumping waste in the oceans and exhaust into the skies; ski resorts and golf courses scar the land and pollute waterways."

I don't necessarily mean to scare you away from these particular industries, because drawbacks can be found in lots of other niches, too. Instead, I just urge you, if you care about investing responsibly, not to assume that some industries are rather clean -- do a little digging first.

If you'd like to learn more about socially responsible investing, here are some articles on the topic:

One efficient way to invest responsibly is to let some smart money managers do so for you -- via socially responsible mutual funds. Shannon Zimmerman recommended one in our Champion Funds newsletter about a year and a half ago, and it's up some 18%, versus 10% for its corresponding index. (Try the newsletter for free and you'll be able to see all our picks and how well they've done.)

Longtime Fool contributor Selena Maranjian owns shares of McDonald's. The Motley Fool has a socially responsible disclosure policy.