Davidson College Department of Biology Guidelines for Using and Citing Sources

Plagiarism--representing another's ideas or words as one's own--is a form of cheating and a violation of Davidson's Honor Code. It is easy to avoid plagiarism with proper citation of sources!

Some aspects of source citation are nuanced. Furthermore, there are variations among different academic fields concerning citation styles and expectations. The purpose of this document is to clarify WHEN and HOW to cite your sources when writing for a biology class. Note that self-plagiarism, or the use of your own previous work in another class, is also inappropriate unless re-use of your work is expressly allowed by the professor. Your biology professors expect original work to be submitted in their classes.

WHEN to cite sources

Direct quotations

If you repeat someone else's words verbatim, enclose those words in quotation marks and provide a citation. For the citation to be complete, there must be two components: a properly formatted in-text citation at the end of the sentence and a corresponding entry in your list of references (see the HOW to cite section below).  Keep in mind that direct quotations are not commonly used in scientific writings such as lab reports or grant proposals but can be appropriate in biology essays and term papers. Direct quotations should flow smoothly within your own thought progression in the assignment; your task is to synthesize the relevant literature, beyond simply compiling direct quotes from sources.


Paraphrasing is restating someone else's ideas while not copying verbatim. There are acceptable and unacceptable ways to paraphrase. Unacceptable paraphrasing includes any of the following: 1) using phrases from the original source without enclosing them in quotation marks; 2) emulating sentence structure even when using different wording; 3) emulating paragraph organization even when using different wording or sentence structure. See examples below. Unacceptable paraphrasing--even with correct citation--is considered plagiarism. When you do paraphrase in an acceptable manner, a proper citation is always required.

Original text

Few laboratory creatures have had such a spectacularly successful and productive history as Drosophila. It first entered laboratories about 1900, revealed its talent for experimental genetics to Thomas Hunt Morgan and his students at Columbia University in the early 1910s, and after some ups and downs in status is still going strong almost a century later. 

(from Kohler, R.E. 1994. The Lords of the Fly. The University of Chicago Press, 321 pages.)

Unacceptable paraphrasing: copying phrases without using quotation marks

Despite some ups and downs in status, nearly a century after the fly revealed its talent to Thomas Hunt Morgan and his students, Drosophila genetics research continues its spectacularly successful history (Kohler, 1994).

(Can you spot the copied phrases? The author of the paraphrasing could avoid trouble by using quotation marks, though it would be much better to write in more original language.)

Unacceptable paraphrasing: emulating sentence structure (here, paragraph structure is also emulated)

No model organism has been so amazingly useful and effective as the fruit fly. The fly came on the scene as an experimental tool at the beginning of the 20th century, was adopted by Thomas Hunt Morgan and his Columbia pupils at Columbia University around 1910, and (despite some fluctuations in attention paid to it) is still a widely used experimental system (Kohler 1994).

(Within each sentence, can you trace the structural similarities?)

Unacceptable paraphrasing: emulating paragraph structure (even though sentence structure is original)

Drosophila is model organism with a rich and useful legacy. Upon arriving on the scene at the turn of the century, the fruit fly soon became the organism of choice for Thomas Hunt Morgan and his Columbia University pupils. Despite fluctuations in status, fly research is still central to the progress of genetics (Kohler, 1994).

(Trace how the ideas flow in a manner identical to the original.)

Acceptable paraphrasing

Thomas Hunt Morgan and colleagues at Columbia University were among the first to use the fruit fly Drosophila as a model organism, adopting it as an experimental system around 1910. Since then, the popularity of the fly has waxed and waned somewhat, but the breadth and depth of current research indicates that Drosophila continues its legacy as an incredibly important research tool (Kohler, 1994).

Describing scientific findings that are not your own

In many forms of scientific writing, you are expected to compile and summarize experimental results from other researchers. Using in-text citations, you must give credit for every mentioned scientific fact that you did not discover yourself, with the exception of facts that are common knowledge, such as "all cells come from pre-existing cells." (If you are not sure what is common knowledge, play it safe and provide a citation; however, if a fact is treated as common knowledge in your source, then you can generally do the same.) Technically, you should provide a citation for each distinct idea, and that sometimes means including one citation in the middle of a sentence and then another citation at the end. However, if a series of sentences clearly go together, with all of them describing findings from a single source, it is acceptable to cite the source only once (instead of after every sentence).

Proposing an idea that is not your own

If you propose a bit of analysis or interpretation that originated in someone else's brain, you must provide a citation.

Using images that you did not create

In your papers and web page assignments, the use of an image (whether photocopied, scanned, or downloaded) that you did not create is the equivalent of a direct quotation. You must include both an in-text citation next to the image and a corresponding entry in your list of references. If you are designing a web page that will be available publicly, you must seek permission to use any borrowed images.

HOW to cite your sources

When you cite a given source, you do so in two places simultaneously. First, a brief in-text citation is placed at the exact sentence or spot in your assignment where the information appears. Second, the full information on the cited source is included as an entry in a list of references at the end.

One common approach is the "author-date" format, in which the in-text citation has one or two author last names plus the publication date. An alternative format is the "numerical" approach, in which the in-text citation is simply a number that is keyed to the list at the end. The author-date format provides more information to the reader, while the numerical format saves space. Here are brief examples, with additional rules and variations below. (Excerpts adapted from Kubisch et al., Cell 96: 437-446 and Mead et al., Science 300: 640-643.)

Author-Date format, in-text component:

Sometimes, depending on the particular mutation, the same gene can be involved in dominant or recessive deafness (Kelsell et al., 1997). Several genes involved in syndromic and nonsyndromic deafness have already been identified and are reviewed in Kalatzis and Petit (1998).

Author-Date format, reference list component:

(Provide complete citations for everything previously cited in-text. Alphabetize the overall list by first author's last name. Do not change the order of authors within a given citation.)

Kalatzis, V., and C. Petit. 1998. The fundamental and medical impacts of recent progress in research on hereditary hearing loss. Hum. Mol. Genet. 7: 1589–1597.

Kelsell, D.P., Dunlop, J., Stevens, H.P., Lench, N.J., Liang, J.N., Parry, G., Mueller, R.F., and Leigh, I.M. 1997. Connexin 26 mutations in hereditary non-syndromic sensorineural deafness. Nature 387: 80–83.

Numerical format, in-text component:

Kuru was the first human prion disease shown to be transmissible, by inoculation of chimpanzees with autopsy-derived brain tissue (1). It is hypothesized that kuru originated from consumption of an individual with sporadic CJD (2), a disease with a remarkably uniform worldwide incidence of around 1 per million and a lifetime risk of around 1 in 50,000.

Numerical format, reference list component:

(Provide complete citations for everything previously cited in-text. Number the entries in the order in which they were cited in the text.)

1. Gajdusek, D. C., C. J. Gibbs, and M. Alpers. 1966. Experimental transmission of a Kuru-like syndrome to chimpanzees. Nature 209: 794-796.

2. Alpers, M., and L. Rail. 1971. Kuru and Creutzfeldt-Jakob disease: clinical and aetiological aspects. Proc. Aust. Assoc. Neurol. 8: 7-15.

Rules and variations for in-text citations

If the source has two authors and you're using the author-date format, list both last names, as in (Roberts and Gonzalez, 1998); if the source has three or more authors, use the format with et al. as in (Jackson et al., 1998). The abbreviation et al. should be italicized, with a period after the al.

If you have already mentioned the authors' names in the sentence, your in-text citation can consist of just a date:
Lopez and Mansouri (1992) demonstrated that the D1S80 marker is not linked to chocolophilia.

The in-text citation can be placed in the middle of a sentence to indicate that only part of the information in the sentence is taken from the source. Example:
The cho gene product metabolizes aromatic amino acids (Tobler et al., 1994) and physically associates with the van gene product (Breyer et al., 1997).

If you cite multiple papers for a single idea, include all in-text citations within one set of parentheses and separate each citation with semi-colon, as in (Tong et al., 1997; Gallagher et al., 1998).

Do not indicate the source page number in your in-text citation.

If you cite a source, you are implying that the idea in question originated in that source. If your source actually cites something else for the same idea, you should track down and cite the original. Only sources that you have examined directly should be included in your reference list. If it is not possible to obtain the original source, you can do the following: (Robertson et al., 1992, as cited in Burgess and Leonard, 2001). Sometimes it is useful to cite a review paper, if there is an appropriate one available--then you don't need to cite as many individual sources. Your in-text citation would read something like (reviewed in Rosenfeld and Lin, 2001). If you don't know how to distinguish a review paper from an original research paper, ask your instructor. A good indicator of an original research paper is a section (often called Materials and Methods, or just Methods) describing specific experimental techniques. Some journals, however, include descriptions of methods in odd places (e.g. embedded in the references, as in Science), so the absence of a Methods section per se is not a sure sign that the paper you're looking at is a review paper.

If you use the numerical approach, the numbers themselves can be enclosed in either parentheses or brackets, or they can be superscripted. Just be consistent throughout the paper.

Rules and variations for reference lists

Citing journal articles
Biologists (unlike in other fields) have no standard way for formatting references. For a given assignment, check the course information for guidance on what reference format to use. If you do not find specific instructions, you will be safe to use the following common format, which is also illustrated in the examples above:
Primary Author's Last Name, Initials, Other Author's Initials, Last Name. Year. Title of article (only capitalize first word). Journal and volume number: page numbers.

In different journals you will see that each publisher seems to have a different style for formatting entries in reference lists. Aspects that vary among different journals include whether:
--initials are before or after an author's last name
--the date is before or after the paper title
--the title is even included (journals like Science and Nature leave it out as a space-saving device)
--the date is enclosed in parentheses
--the journal name is italicized
--the volume number is italicized or in bold face
--there is a colon or comma after the journal volume number
--the issue number (in addition to the volume number) is included
--the whole page range or just the first page number is listed
--different components (e.g. authors, title, journal, etc.) are in boldface.

Whatever format you use, you should be completely consistent in how you format your list of references.

Citing books
Use the following format to cite a whole book:
Author Last Name, Initials. Date. Title. Publisher, City, number of pages.
Use this format for citing book chapters:
Author Last Name, Initials. Date. Title of chapter. In: Title of book (editors). Publisher, City, number of pages.

Citing web sites
If you obtain from the internet a journal article originally published in a print journal, cite the article exacly as you would for a print source. If you obtain an peer-reviewed article from one of the growing number of online-only journals, cite the article as for a print source, but also append URL. Accession Date.

Most other web sites are not peer-reviewed, and so their reliability is highly variable. It is rare to use such non-peer-reviewed web sites as sources for scientific writing. For any given assignment, check your course syllabus or assignment handout to see if you are permitted to use such sources. If so, then it is still up to you to evaluate the reliability of a web site; consider the credentials of the author, the purpose of the web page, and the date of the last revision, among other things.

To cite a web site, use the following reference format:
Author Last Name, Initials. Date page created or revised. Title of page. Title of larger work if applicable. URL. Accession date.

Please confer with your professor and/or with tutors in the Math and Science Center to cover any questions you may have! We are happy to help.