Q: I saw a post online that said only people over the age of forty put two spaces after a period. But I’m under thirty, and my college professors said to use two. I’m confused. Which is correct?
A: Both are correct in different circumstances.
As with so much else that publishing professionals get needlessly worked up about, this is a style choice, not a matter of right or wrong.
Three of the most popular style books currently in use, The Chicago Manual of Style, The Associated Press Stylebook, and The MLA Handbook (Modern Language Association), all call for one space after what we call terminal punctuation—that is, whatever marks the end of a sentence, whether it’s a period, question mark, or exclamation point.
The Publication Manual of the American Psychological Association, in its current edition (the sixth), specifies two spaces after terminal punctuation. This is a change from, at least, the fourth and fifth editions. If anyone has access to earlier editions, I’d be interested in finding out what the style was prior to the fourth edition. You can find a discussion of this change on the APA Style blog. The change was ostensibly made to improve readability. As you’ll see from the comments on the blog, whether the change accomplishes this goal is debatable.
What is not debatable is that if your professor tells you to prepare your papers a certain way, you do as they tell you. Even the MLA includes a note that one space should be used unless your instructor tells you otherwise. That this allowance is made and that APA imposed a two-space guideline after having a one-space guideline for at least fifteen years tells me a lot of professors want to see two spaces. Give the person who is grading your paper what they ask for.
APA style is used at many colleges in many disciplines other than psychology. You can find a summary of APA style guidelines at the Purdue OWL website. Since many students use this manual, it is inappropriate to frame the argument—and unfortunately, especially online, this topic does devolve into ridiculous arguments—as one about age or whether one learned to type on a typewriter or not.
The only consideration one must make on this topic is what style you have been instructed to follow. If your professor says use APA style, then put two spaces after terminal punctuation. If you’re submitting an article to a newspaper that follows AP style or to a book publisher that adheres to Chicago style, use one.
If you are accustomed to typing your sentences a particular way and don’t want to re-learn a different style for different venues, replacing APA-style spacing with Chicago-style spacing (or vice versa) is easy enough to do once you’ve finished writing.