: Impact on SEO of adding categories/tags in front of the HTML title Possible Duplicate: Does the order of keywords matter in a page title? All StackExchange sites add the most-used

Possible Duplicate:
Does the order of keywords matter in a page title?

All StackExchange sites add the most-used tag of a question in front of the HTML title for SEO purposes. On Stackoverflow for example this is usually the programming language, so you end up with a title like

python - How do I do X?

This has obviously an enourmous benefit on SEO as the programming language is an extremely important keyword that is very often omitted from the title.

Now, my question is for the cases where the tag isn't an important keyword missing from the title, but just a category. So on Biology.SE for example one would have questions like

biochemistry - How does protein X interact with Y?

or on Skeptics

medical science - Do vaccines cause autism?

Those tags are usually not part of the search terms, they serve to categorize the content but users don't use those tags in their searches.

How harmful is adding tags that are not used in searches in terms of SEO? Is there any hard data on the impact this practise might have on SEO?

The negative aspects I can imagine, but have no data to show that it is actually a problem are:

I heard that search engines dislike keyword stuffing and this might trigger some defense mechanisms against that
It's a practise associated with less reputable sites, a keyword in front that doesn't fit the actual title well might look suspicious to some users.
It wastes precious space in the title shown in search results.