The first heading level provides labels for the main sections of your paper, such as "Method," "Results" and "Discussion." If your paper contains multiple chapters, use this heading level to title the separate chapters. If your paper presents multiple separate experiments, then use this heading level to title each experiment. Center the first-level headings between the left and right margins, without any indentation, and use boldface text. Capitalize the first letter of each main word and of any word longer than three letters, even prepositions. Press "Enter" or "Return" after finishing the heading, and begin the next paragraph on the next line.
Use the second heading level for the main subsections under each primary section. For example, the "Methods" section might contain subsections entitled "Participants," "Materials," "Design" and "Procedure." If you divided your paper by chapters or experiments, then use this second heading level to present the main elements of each chapter or experiment. Align this heading level with the left margin, and write it in boldface text. Use uppercase and lowercase capitalization, as in the first level. After the heading, begin the next paragraph on the next line.
Third-level headings mark subsections within subsections; for example, if the Materials portion of your Methods section contains multiple parts, you can use this level to divide and label those parts. Indent this heading half an inch from the left margin, and write it in boldface text. Capitalize the first letter of the first word, and the first letter of any proper nouns, but write the rest of the words in all lowercase letters. At the end of the heading, type a period. Type a space, and then immediately begin the normal text of this subsection in plain text, not in boldface. Don't start a new line.
Use the fourth heading level to organize subsections that fall within a third-level section. Format this level exactly like the third level: Indent it 1/2 inch, only capitalize the first word and proper nouns, write it in boldface text, and end it with a period. In addition, to distinguish it from the third level, italicize the entire heading. Type a space, and then immediately begin the normal text of this subsection in plain text. Don't start a new line.
The fifth level, usually only used in very long and complex papers like theses and dissertations, is the last level of headings used in APA papers. To format a fifth-level heading, indent it 1/2 inch. Italicize the text, but don't use boldface. Only capitalize the first word and any proper nouns. At the end of the heading, add a period. Type a space, and then immediately begin the normal text of this subsection, in plain text, and don't start a new line.
In APA style, the introduction doesn't need a heading. Instead, readers simply assume that the first section of your paper presents the introduction. Accordingly, the APA manual specifically prohibits giving the introduction any heading.