1. Ethical Principles of Psychologists and Code of Conduct (2002) -- http://www.apa.org/ethics/code/index.aspx#8_11
Plagiarism and Publication Credit are addressed by the American Psychological Association in its ethics code.
Psychologists do not present portions of another's work or data as their own, even if the other work or data source is cited occasionally.
8.12 Publication Credit
(a) Psychologists take responsibility and credit, including authorship credit, only for work they have actually performed or to which they have substantially contributed. (See also Standard 8.12b, Publication Credit.)
(b) Principal authorship and other publication credits accurately reflect the relative scientific or professional contributions of the individuals involved, regardless of their relative status. Mere possession of an institutional position, such as department chair, does not justify authorship credit. Minor contributions to the research or to the writing for publications are acknowledged appropriately, such as in footnotes or in an introductory statement.
(c) Except under exceptional circumstances, a student is listed as principal author on any multiple-authored article that is substantially based on the student's doctoral dissertation. Faculty advisors discuss publication credit with students as early as feasible and throughout the research and publication process as appropriate. (See also Standard 8.12b, Publication Credit.)
2. Publication Manual of the American Psychological Association, 6th edition
These two principles are discussed on pages 15-16 of the new APA style manual (2010), and Chapter 6 is devoted to Crediting Sources. Quotation marks are suggested when using the exact words of another. It also stresses that not only must you credit a source in the text when you paraphrase an author, but you must do so every time you paraphrase. These rules apply also to personal communications, such as notes taken in class lectures, face-to-face conversation, emails, etc.
If a written work is intended for publication, the APA also requires proof of permission to use material from copyrighted sources. This permission must be footnoted for material that is quoted at length, and the original author’s letter of permission must accompany the manuscript. This rule applies to figures and tables as well, and credit to the author or copyright holder must be included in the caption.