user948: With the site's graduation and increase in low-rep visitors, some of whom are beginning to answer, I (and many others) have had to comment a number of times that answers need to be supported by credible citations, else they risk challenge and removal. This is often when the poster is just throwing ideas around, mixing research and personal experience, making things up, handwaving, etc.
However, I just came across this answer, where a very high-rep user (not that it necessarily matters) gave a quick explanation to a simple enzyme question, using facts that anyone conversant in the field of biochemistry should know to be supported by reams of literature, and accepted as fact in just about every biochem textbook in the world. However, a user challenged him to support his answer with a citation, claiming

Any layman statement cannot be accepted as an answer, no matter how general information it is.

Now, it should be fairly easy to pull a Wikipedia link out of somewhere, but that's not my main issue. What I'm asking is: When does a statement in an answer become sufficiently complex that it requires a citation? For example, if I state in an answer to an immunology question that B cells are part of the immune system and produce antibodies, do I need to cite that? In a genetics question, do I need to support a statement that DNA is the heritable material that carries our genes from one generation to the next, and codes those genes in three-base codons? Where is the line?

user1590: Citations and external material serve two purposes, to support claims and statements made in the post, and to allow easier follow-up reading by the readership. At the end of the day, we should be willing and able to support the validity of any statement we make in an answer, so we should bear that in mind when we write answers. For example, one could say:

DNA is a nucleic acid composed of nucleotides.

Now a citation isn't necessary, because it's not really a contentious point. However, a link to say, the wikipedia article on DNA, would support that claim while also producing material that readers can use for further information should they want it. Therefore, a reference is often not necessary but would be beneficial.
One could instead say:

DNA is made of balloons.

This statement would benefit from having a citation to support that claim, because it is likely to be contentious. Obviously there is a scale, this is not a binary problem, some points will be more contentious than others. Ideally the person writing the answer would give a good reference to support this statement, such as a proper scientific paper, as a proactive referencing measure. However, often such contentious statements are poorly supported, or entirely unsupported statements. This will likely lead to comments along the lines of:

WTF?! DNA is not made of balloons! Please could you provide a reference to support your claim?

As a result, the provider of the answer should either edit to remove the claim or provide supporting evidence; reactive referencing. If someone writes something that they are not willing or able to defend or support then they should remove that text from their answer (or it should be removed for them).
As a general rule of thumb, I would say that referencing is not always necessary but we should 
i) Always produce answers that could be supported if requested
ii) Use a generous serving of external links to help the user find definitions, fill gaps in knowledge that may help them understand the answer, or allow them to follow up with further reading 
iii) Try to support any material that is likely to be either contentious in its validity or not well known with proactive referencing
iv) Provide reactive references to support claims if other users call you out on them. If you can not provide a reference you should edit or delete the material to make it fall in line with accepted or supported consensus
So while we should be striving to be proactive in our referencing, we should at the very least be willing to be reactive.

What to do about poorly referenced answers?
i) Comment and request references, ideally drawing attention to specific points of contention, or areas where the readership would likely benefit from external material
ii) Add references yourself to other peoples answers if you think you can provide the material needed. For example, you might see someone talking about the mechanisms of evolution, so you could add the links for the Understanding Evolution pages
iii) Downvote and tell the user you are downvoting due to the lack of references, again being specific about details you want supported
iv) Flag for moderator attention so they can add this banner which will warn other users that material is disputed.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

Note that comments of "could you add some references to your answer" are useful, and indicate that the answer is generally in need of supporting material, but we also should try to be specific wherever possible, e.g. "could you add some references to your answer, especially to support your claim that DNA is made from balloons"
Unfortunately, there's not a lot else we can do about poorly referenced answers. If the writer of the answer does not want to provide references we can't force them. I've started a discussion thread on counter-measures.

user3340: My opinion on when is citation needed.
References are always appreciated but are strictly required in certain cases such as:

Answers stating non-obvious facts and recent experimental findings.
Answers related to a popular topic. (health related topics, evolution, wildlife conservation, etc). This is required in order to avoid any misinterpretation by non-experts.
Species identification (photo-citation). This may not be always possible but if not provided with a valid picture citation then the answer is only as good as a comment.

Citations for the first two kind of questions (preferably all questions) should consist of either/some/all of these (listed as per the order of importance):

Peer-reviewed articles
Books
Informative websites hosted by educational foundations, research organizations and universities.
Well referenced/elucidated encyclopedia. A stub wikipedia article is not a good reference. 

Popular-science blogs, in my opinion are not very trustworthy. Even if they are not incorrect they tend to oversimplify/exaggerate the original findings.
Citations to peer-reviewed articles should generally be made using their DOI name or pubmed accession. Book citations should mention the ISBN. (For books from NCBI bookshelf, there is an option to generate citation in the right panel). 
Citations are not needed for answers that are:

Addressing homework questions.
Deduced from basic principles of biology/mathematical logic.
About troubleshooting/designing experiments
Suggesting bioinformatics/analytical approaches (for e.g. how to process/interpret some data)

When providing answers to very basic (textbook level) question it is good to provide a wikipedia link or a "soft" citation of a textbook (for instance "you can read more about this from - Molecular Biology of the Cell by Alberts et al.").

user3553: Don't cite everything.
In my undergraduate honours degree I came from the other end of the stick and referenced too much; an equally unhelpful filter. I cited almost every sentence and statement with many sources, even if a study simply mentioned an established fact that I was stating, I referenced it. The feed-back, which I now go by, was:

Only cite the specific studies that describe novel findings; general "historical" dogmas and rules don't require citation.

I did a bit of googling and there isn't much useful information regarding when to reference. Here is a piece of advice from the University of Reading:

... every time you use or refer to an idea or piece of information
  that you learnt from a text, you should include a reference to the
  source.

If someone is describing methods, this is what Nature protocols states:

Include references to key papers where the protocol has been used previously, including those published by your own group or to reviews that discuss applications of the protocol (see below for how to cite references).

Personal view on Bio.SE
I think that on our site, there are two reasons why I want to see citations.

I don't trust you. These Q&As are just written by internet folk. Using citations allows users to check if that internet user is right or wrong according to an external source, and the quality of the source allows us to validate how seriously a statement should be taken. General underpinnings shouldn't be cited since they go without saying. If someone from another field of Biology wouldn't be familiar with the idea, link to somewhere even if it's informal. If the subject is a novel idea or has only recently been shown, then it's probably appropriate to show the peer reviewed source.

I would not want you to cite a body of work to show evolution happens.
I would want you to cite a wikipedia article describing how neo-darwinism can explain the earth's history.
I would want you to cite a journal article that shows a predisposition to AT mutations in bacteria.
If these aren't cited appropriately I'd be tempted to downvote and explain why.

I want more information. We're all here to learn something. This point relates to informal citations like the wikipedia example above. Sources allow us to go into more depth and to get to interesting content that experts have more readily available than google.

I would be pleased if you would cite an interesting documentary on cordyceps.
I would not care if you were to cite wikipedia "Fungi".
I wouldn't down vote in the absence of these interesting tit-bits, but it's a productive way to make me want to upvote.



948: <p>With the site's graduation and increase in low-rep visitors, some of whom are beginning to answer, I (and many others) have had to comment a number of times that answers need to be supported by credible citations, else they risk challenge and removal. This is often when the poster is just throwing ideas around, mixing research and personal experience, making things up, handwaving, etc.</p>

<p>However, I just came across <a href="https://biology.stackexchange.com/a/43750/948">this answer</a>, where a very high-rep user (not that it necessarily matters) gave a quick explanation to a simple enzyme question, using facts that anyone conversant in the field of biochemistry should know to be supported by reams of literature, <em>and</em> accepted as fact in just about every biochem textbook in the world. However, a user <a href="https://biology.stackexchange.com/questions/43749/competitive-inhibition#comment75804_43750">challenged</a> him to support his answer with a citation, claiming</p>

<blockquote>
  <p>Any layman statement cannot be accepted as an answer, no matter how general information it is.</p>
</blockquote>

<p>Now, it should be fairly easy to pull a Wikipedia link out of somewhere, but that's not my main issue. What I'm asking is: When does a statement in an answer become sufficiently complex that it requires a citation? For example, if I state in an answer to an immunology question that B cells are part of the immune system and produce antibodies, do I need to cite that? In a genetics question, do I need to support a statement that DNA is the heritable material that carries our genes from one generation to the next, and codes those genes in three-base codons? Where is the line?</p>
 1590: <p><strong>Citations and external material serve two purposes</strong>, to support claims and statements made in the post, and to allow easier follow-up reading by the readership. At the end of the day, <strong>we should be willing and able to support the validity of <em>any</em> statement we make in an answer</strong>, so we should bear that in mind when we write answers. For example, one could say:</p>

<blockquote>
  <p>DNA is a nucleic acid composed of nucleotides.</p>
</blockquote>

<p>Now a citation isn't necessary, because it's not really a contentious point. However, a link to say, the wikipedia article on DNA, would support that claim while also producing material that readers can use for further information should they want it. Therefore, a reference is often not necessary but would be beneficial.</p>

<p>One could instead say:</p>

<blockquote>
  <p>DNA is made of balloons.</p>
</blockquote>

<p>This statement would benefit from having a citation to support that claim, because it is likely to be contentious. Obviously there is a scale, this is not a binary problem, some points will be <em>more</em> contentious than others. Ideally the person writing the answer would give a good reference to support this statement, such as a proper scientific paper, as a <strong>proactive referencing</strong> measure. However, often such contentious statements are poorly supported, or entirely unsupported statements. This will likely lead to comments along the lines of:</p>

<blockquote>
  <p>WTF?! DNA is <em>not</em> made of balloons! Please could you provide a reference to support your claim?</p>
</blockquote>

<p>As a result, the provider of the answer should either edit to remove the claim or provide supporting evidence; <strong>reactive referencing.</strong> If someone writes something that they are not willing or able to defend or support then they should remove that text from their answer (or it should be removed for them).</p>

<p><strong>As a general rule of thumb, I would say that referencing is not always necessary but we should</strong> </p>

<p><em>i)</em> Always produce answers that could be supported <em>if requested</em></p>

<p><em>ii)</em> Use a generous serving of external links to help the user find definitions, fill gaps in knowledge that may help them understand the answer, or allow them to follow up with further reading </p>

<p><em>iii)</em> Try to support any material that is likely to be either contentious in its validity or not well known with proactive referencing</p>

<p><em>iv)</em> Provide reactive references to support claims if other users call you out on them. If you can not provide a reference you should edit or delete the material to make it fall in line with accepted or supported consensus</p>

<p><strong>So while we should be striving to be proactive in our referencing, we should at the very least be willing to be reactive.</strong></p>

<hr>

<p><strong>What to do about poorly referenced answers?</strong></p>

<p><em>i)</em> Comment and request references, ideally drawing attention to specific points of contention, or areas where the readership would likely benefit from external material</p>

<p><em>ii)</em> Add references yourself to other peoples answers if you think you can provide the material needed. For example, you might see someone talking about the mechanisms of evolution, so you could add the links for the <a href="http://evolution.berkeley.edu/evolibrary/home.php" rel="nofollow noreferrer">Understanding Evolution</a> pages</p>

<p><em>iii)</em> Downvote and tell the user you are downvoting due to the lack of references, again being specific about details you want supported</p>

<p><em>iv)</em> Flag for moderator attention so they can add this banner which will warn other users that material is disputed.</p>

<blockquote>
  <p>Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.</p>
</blockquote>

<p>Note that comments of <em>"could you add some references to your answer"</em> are useful, and indicate that the answer is generally in need of supporting material, but we also should try to be specific wherever possible, e.g. <em>"could you add some references to your answer, especially to support your claim that DNA is made from balloons"</em></p>

<p>Unfortunately, there's not a lot else we can do about poorly referenced answers. If the writer of the answer does not want to provide references we can't force them. <a href="https://biology.meta.stackexchange.com/questions/3345/an-effective-measure-to-counter-unreferenced-answers">I've started a discussion thread on counter-measures.</a></p>
 3340: <p>My opinion on when is citation needed.</p>

<p>References are always appreciated but are strictly required in certain cases such as:</p>

<ul>
<li>Answers stating non-obvious facts and recent experimental findings.</li>
<li>Answers related to a popular topic. (health related topics, evolution, wildlife conservation, etc). This is required in order to avoid any misinterpretation by non-experts.</li>
<li>Species identification (photo-citation). This may not be always possible but if not provided with a valid picture citation then the answer is only as good as a comment.</li>
</ul>

<p>Citations for the first two kind of questions (preferably all questions) should consist of either/some/all of these (listed as per the order of importance):</p>

<ul>
<li>Peer-reviewed articles</li>
<li>Books</li>
<li>Informative websites hosted by educational foundations, research organizations and universities.</li>
<li>Well referenced/elucidated encyclopedia. A stub wikipedia article is not a good reference. </li>
</ul>

<p>Popular-science blogs, in my opinion are not very trustworthy. Even if they are not incorrect they tend to oversimplify/exaggerate the original findings.</p>

<p>Citations to peer-reviewed articles should generally be made using their <a href="https://www.doi.org/" rel="nofollow">DOI name</a> or <a href="http://www.ncbi.nlm.nih.gov/pubmed" rel="nofollow">pubmed accession</a>. Book citations should mention the <a href="https://en.wikipedia.org/wiki/International_Standard_Book_Number" rel="nofollow">ISBN</a>. (For books from NCBI bookshelf, there is an option to generate citation in the right panel). </p>

<p>Citations are not needed for answers that are:</p>

<ul>
<li>Addressing homework questions.</li>
<li>Deduced from basic principles of biology/mathematical logic.</li>
<li>About troubleshooting/designing experiments</li>
<li>Suggesting bioinformatics/analytical approaches (for e.g. how to process/interpret some data)</li>
</ul>

<p>When providing answers to very basic (textbook level) question it is good to provide a wikipedia link or a "soft" citation of a textbook (for instance "<em>you can read more about this from - Molecular Biology of the Cell by Alberts et al.</em>").</p>
 3553: <h1>Don't cite everything.</h1>

<p>In my undergraduate honours degree I came from the other end of the stick and referenced too much; an equally unhelpful filter. I cited almost every sentence and statement with many sources, even if a study simply mentioned an established fact that I was stating, I referenced it. The feed-back, which I now go by, was:</p>

<blockquote>
  <p>Only cite the specific studies that describe novel findings; general "historical" dogmas and rules don't require citation.</p>
</blockquote>

<p>I did a bit of googling and there isn't much useful information regarding when to reference. Here is a piece of advice from the <a href="https://www.reading.ac.uk/internal/studyadvice/StudyResources/Reading/sta-references.aspx" rel="nofollow">University of Reading</a>:</p>

<blockquote>
  <p>... every time you use or refer to an idea or piece of information
  that you learnt from a text, you should include a reference to the
  source.</p>
</blockquote>

<p>If someone is describing methods, this is what <a href="http://www.nature.com/nprot/info/gta.html" rel="nofollow">Nature protocols</a> states:</p>

<blockquote>
  <p>Include references to key papers where the protocol has been used previously, including those published by your own group or to reviews that discuss applications of the protocol (see below for how to cite references).</p>
</blockquote>

<h1>Personal view on Bio.SE</h1>

<p>I think that on our site, there are two reasons why I want to see citations.</p>

<ol>
<li><p><strong>I don't trust you.</strong> These Q&amp;As are just written by internet folk. Using citations allows users to check if that internet user is right or wrong according to an external source, and the quality of the source allows us to validate how seriously a statement should be taken. General underpinnings shouldn't be cited since they go without saying. If someone from another field of Biology wouldn't be familiar with the idea, link to somewhere even if it's informal. If the subject is a novel idea or has only recently been shown, then it's probably appropriate to show the peer reviewed source.</p>

<ul>
<li>I would not want you to cite a body of work to show evolution happens.</li>
<li>I would want you to cite <a href="https://en.wikipedia.org/wiki/Modern_evolutionary_synthesis#Understanding_of_Earth_history" rel="nofollow"><strong>a wikipedia article</strong> describing how neo-darwinism can explain the earth's history</a>.</li>
<li>I would want you to cite <a href="http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1001115" rel="nofollow"><strong>a journal article</strong> that shows a predisposition to AT mutations in bacteria</a>.</li>
<li>If these aren't cited appropriately I'd be tempted to downvote and explain why.</li>
</ul></li>
<li><p><strong>I want more information.</strong> We're all here to learn something. This point relates to informal citations like the wikipedia example above. Sources allow us to go into more depth and to get to interesting content that experts have more readily available than google.</p>

<ul>
<li>I would be pleased if you would cite <a href="https://www.youtube.com/watch?v=XuKjBIBBAL8" rel="nofollow">an interesting documentary on cordyceps</a>.</li>
<li>I would not care if you were to cite wikipedia "Fungi".</li>
<li>I wouldn't down vote in the absence of these interesting tit-bits, but it's a productive way to make me want to upvote.</li>
</ul></li>
</ol>
