Misunderstanding of or confusion about proper capitalization plagues many writers. Even an accomplished editor can’t often revert to what “looks right” or “feels right” because we as a culture are so confused by capitalization that we often see published work in which the capitalization is wrong. In addition, different publications have different preferences when it comes to capitalization, and many publishers make conscious choices to break the rules for stylistic or political reasons. So, rather than paging through the whole Chicago Manual of Style or the stylebook of your choice, I’m going to lay out three of the most common capitalization errors, along with the correct style.
- Professional Titles – As tempting as it is, your job title does not get capital letters unless your name is attached to it. For example, “Doctor Zhivago” has capitals, but “the doctor” does not. Barack Obama is the president, which means he is President Barack Obama. I used to be the managing editor of NewMoon.com. I liked to refer to myself as the Managing Editor, but as such, I wasn’t a great editor, as that capitalization was incorrect. (CMS: 7.22)
- Medical Conditions – Medical conditions do not receive capitalization unless they are named after someone, in which case only the name is capitalized and not the whole condition. For example, only the D in Down’s is capitalized when referring to Down syndrome, not the S in syndrome. Specific–but still generic–conditions do not receive capitalization: bipolar disorder, heart disease, alcoholism. (CMS: 7.117)
- Terms of Endearment – Most sources will say that terms of endearment, when used in place of someone’s name, do not get capitalized: “Will you get that for me, dear?” This one is easy to mess up because when using a family name, such as mom or dad, in place of a name, it does get capitalized: “Are we there yet, Mom?” This lack of clarity may be why you’ll get an array of answers if you Google “capitalize terms of endearment.” The style guides I have at my disposal don’t mention terms of endearments at all. If you have a definitive source on this rule, please leave a comment and enlighten me!
Due to the vast array of stylistic preferences or the potential difficulty of finding a reliable source for any given capitalization rule, you’ll see lots of items capitalized every which way. In the end, the most important thing is consistency–but consistency is even better when it’s consistently right.
LL Word 
I’ve always struggled with capitalization of species names. I really want to capitalize them, but apparently it’s incorrect. The rules are the same as medical conditions. You can only capitalize names and places. Valid examples are American ginseng, Maximilian’s sunflower, and narrow-leaved purple cornflower. Just to make things confusing, some species that have a single accepted common name, such as birds and fish, are often allowed to be capitalized.
Yes, species names (and plant names in general) are SO confusing. My style guide says that genus (and anything higher) gets capitalized, while species (and anything lower–is there anything lower? subspecies, perhaps?) don’t, not even when using a proper noun. Hm, that seems a little fishy. I always find myself wanting to capitalize even the vernacular names for plants, especially trees: Oak tree, Maple tree. But I guess if I do that, I might as well start capitalizing words like Cat and Cow. The GOOD news is that a lot of publications create their own style guides, so all you have to do is create your own publication, and you can pretty much do whatever you want grammatically–as long as you do it consistently. 😉
Yep, on scientific names, the genus is capitalized and everything below it is lower case. Subspecies and variety are the most common things below species. I believe scientific names are always supposed to be italicized too. On top of that, I think you are supposed to include the authors of a scientific name, but I usually ignore that rule. So, the wood anemone I saw last weekend would be Anemone quinquefolia L. var. bifolia Farw. with the genus, species and variety italicized, but not the authors.
Keir, for some reason I can’t respond to your comment directly; I wonder if WordPress only allows short threads? Anyway, I’m replying to myself in this thread instead just to say that you’re correct in that the Latin names of plants are supposed to be italicized. Including the authors as well does make things get a bit clunky.
p.s. It is Down syndrome, not Down’s syndrome 🙂
p.p.s. Both are correct.
My current pet peeve about capitalization is seeing capitalization of Every Single Word In A Sentence. I have to correct a lot of things that look like that for work. It mystifies me that someone out there thinks that is correct (and it is even greater mystery that someone thinks it looks good!)
You know, when I was doing some research for this post, I found one site that said in the past, people used to capitalize every noun in a sentence. Maybe those people have been reading old stuff like that! I’ve heard that in some languages, people capitalize the first letter in words they want to stress, much like we use italics. Sometimes when I see weird capitalization patterns, I wonder if the person is writing in English as a second language from a language that is more liberal with capitalization. The other possibility is that those people who capitalize every word just read nothing but titles. :p
I doubt they’ve been reading old stuff like that, nor that they speak other languages. It looks like it is done for emphasis, like a title. Yet instead of it being a title, it is an entire paragraph.
(And I didn’t mean to imply that it is one person doing it, because I’ve seen it a lot on what are supposed to be professional websites.)
Chicago Manual of Style says terms of endearment are not used as a name replacement, like ‘mom’ or ‘dad’ are used as name. Rather, terms of endearment are meant as affections, vocative names, and therefore are not capitalized.