Kessler (2003) found that among epidemiological samples

Early onset results in a more persistent and severe course (Kessler, 2003).

If the author’s name is mentioned in the text as part of the narrative, you do not need to include the year for all subsequent mentions of the text unless you use a parenthetical citation. If you use a parenthetical citation with both author name and date in parentheses, then all subsequent mentions include the author name and year.

Author name not in parentheses

Among epidemiological samples Kessler (2003) found [some really bad stuff]. Kessler also found….

But

Author name in parentheses

Early onset results in a more persistent and severe course (Kessler, 2003). Kessler (2003) also found.   

Use the first few words found in the reference list--usually the title. Italicize articles, chapters, and webpages. Use italics for larger works. If the work is credited to Anonymous, use it as the author.

(“Study finds,” 2007)

(Anonymous, 2008)

Citation with two authors

Kurtines and Szapocznik (2003) demonstrated

(Jöreskog & Sörbom, 2007)

Citation with three or more authors

First mention for three, four, or five authors

Kisangau, Lyaruu, Hosea, and Joseph (2007)

Subsequent mentions

Kisangau et al. (2007)

Six or more authors

Include only the name of the last author followed by et al. in all mentions.

Wasserstein et al. (2005)

First mention

National Institute of Health (NIMH, 2003)

Subsequent mentions

(NIMH, 2003)

List two or more works by different authors who are cited within the same parentheses in alphabetical order. Separate the citations using semi-colons. 

Several studies (Miller, 1999; Shafranske & Mahoney, 1998)

(V.G. Nguyen, personal communication, September 28, 1998)