In APA style, you should center the primary headings---the titles of the paper's main sections---in order to demonstrate their importance visually. All other text in the body of an APA paper, except in figures and tables, aligns to the left of the page. Hence, the centered heading stands out as an anomaly, drawing the reader's eye and suggesting that it forms an important marker. Even if the reader doesn't consciously acknowledge the heading's significantly differentiated format, its deviation from the norm creates an intuitive impression.
Centering the headings of the most important sections enhances your paper's organization. Because you center only the first level of headings, these headings help the reader keep track of your paper's main sections. Research studies tend to fill many pages, so if your paper lacked noticeable headings, your readers could lose sight of the paper's direction or overall arrangement. As the Writing Center at the University of Wisconsin explains, headings help readers understand which parts of a paper matter the most significantly and which simply contribute to larger segments.
General academic style uses centered primary headings. By following this convention and by setting its own standard for centered headings, APA style helps maintain a specified format among academic papers and scholarly journals. Within academia and the realm of research, most papers adhere to this rule; this helps students and scholars follow others' papers accurately and create new papers that others, in turn, can conveniently comprehend. According to the American Psychological Association, the APA rules for formatting headers and other paper elements help "ensure clear and consistent presentation of written material."
The previous edition of the APA style manual instructed writers to center as many as three levels of headings, forming a system that minimized the significance of centering the main headings. This previous version also presented a complex method for selecting the appropriate levels for each paper; for example, a paper that contained only two levels should use levels 1 and 3, rather than the more intuitive 1 and 2. In the style manual's 6th and newest edition, the APA updated the heading rules to ease this process for scholars and to enhance the format's appearance in online publications. Now, a two-level paper simply uses the first two levels. Moreover, in any paper---regardless of its number of levels---you should only center the primary, first-level headings.
Bold each centered heading, but don't underline it. Capitalize the first letter of each major word, but write articles and small prepositions in all lowercase letters. Don't add a letter or number to the heading, warns the Purdue Online Writing Lab; the APA heading levels maintain a clear system of organization, so you don't need to number or otherwise seriate the headings.