The instructor of the course corrects you: "'He or she has', not 'they have'." How do you feel about being corrected in this way? Do you think the instructor was justified?
Just as it is normal to evaluate the religious beliefs and practices and the family behavior of particular individuals within our culture, it is also normal to evaluate the linguistic behavior of people. We may treat some speech and writing patterns as acceptable or unacceptable, superior or inferior, appropriate or inappropriate. Attitudes such as these must be based on a standard, some idea of what counts as desirable behavior. People who are concerned with defining and maintaining linguistic standards are prescribing (rather than describing) language. Usually they prescribe aspects of grammar, and in this role they are referred to as prescriptive grammarians. There are three kinds of questions we can ask of linguistic standards and language prescription.
Why a standard
One reason for a linguistic standard is that people within a community (for example, a nation) will be better able to understand one another if they agree on a set of words and a set of rules for pronunciation, spelling, and grammar. The process of defining the rules (and sometimes the set of words as well) is called standardization. Once a standard has been agreed on, it can be used in the media and taught in the schools.
Japan provides a good example of this process of standardization and promulgation of the standard. Children all over Japan, speaking widely divergent dialects of Japanese, are all taught to speak and write Standard Japanese in school, and radio and television announcers are all expected to be familiar with the standard vocabulary, pronunciation, and grammar. If these announcers were to speak in the southern dialect of Kagoshima or the northern dialect of Aomori, people in most of Japan would have difficulty understanding them. Something like this happens in the United States, though more informally. Children across the country are taught the same written standard vocabulary and grammar, though they may not be taught a single standard pronunciation, and radio and television announcers tend to speak in a single standard.
A related, though often unstated, goal of a standard may be to eliminate diversity, which may be perceived as a threat to national unity. Sometimes the diversity is reflected in different, though closely related, dialects, but sometimes it is reflected in completely different languages. It is one thing to teach a standard in the schools across a country. It is another to discourage people from speaking their local dialects or languages (for more on dialects and languages, see the next section). Sometimes the native speakers of the non-standard dialects or languages become willing participants in this process in their desire to be integrated into the society.
In Japan, for example, standardization sometimes had the effect of eliminating diversity. In the case of the Ryukyu Islands, the southernmost region of Japan, the dialects that are spoken there are so different from the standard that they may be considered a separate language (or languages). Today, following the repressive policies of school administrators in the first half of the twentieth century as well as the economic and social pressures for Ryukyuans to conform, this language is on the verge of extinction. This is a familiar phenomenon in many countries with minority populations. In the US and Canada, immigrant and American Indian students have often been prevented from speaking their home languages in schools. These educational policies were most notorious in the case of American Indian students, who were sometimes sent to schools which kept them separated from their communities and were punished for speaking in their native languages. These policies were one of the major factors leading to the extinction and near extinction of the majority of North American Indian languages. For more on the topic of language death and "endangered languages", that is, languages in danger of dying, see the websites of Terralingua and the Foundation for Endangered Languages, two organizations dedicated to linguistic diversity.
Standard languages and politics
Clearly language standardization is a political issue, and as such it is not really the business of linguists (though it is studied by sociologists interested in the social and political aspects of language). However, as linguists often become closely involved with the people whose languages they study, they may become advocates for these groups when their languages or their well-being are threatened because of the language policies of governments.
Other reasons are often stated for prescribing language, though these may mask the ultimate political reasons. These reasons include the supposed illogical or ambiguous nature of some constructions used by people, and linguists have also sometimes gotten involved in the debate in these cases because of their expertise. An example is the use of English they, them, and their to refer to a single person, usually of unknown or unspecified gender, as in the example in the box above. The idea is that since they is supposed to be plural, it should not be allowed to refer to one person. In cases like this, prescriptive grammarians are trying to actually improve the language or perhaps to preserve what they suppose to be an earlier, purer form of the language. The problem is that linguists and other language scientists, experts on the "logic" and the degree of ambiguity in language, can find nothing inherently wrong with any of these constructions, at least not with the usual examples of "bad" English constructions. In fact languages seem to have their own built-in "prescriptive" mechanism which weeds out whatever patterns don't work. It's a survival-of-the-fittest sort of arrangement, and it implies that those common patterns such as "singular they" (which has been around in the spoken and written language for at least 500 years) are quite fit linguistically.
Where standards come from
If we are to define a set of standards for a community, they have to come from somewhere. One possibility for the source is from other languages, and, strange though this may seem, standardizers have sometimes resorted to it. Take the "split infinitive", which prescriptive English grammarians sometimes argue against. An English infinitive is an expression consisting of to followed by a verb stem (don't worry now if you don't know what this is), for example, to go or to sing. So according to this rule, it is incorrect to say or write to boldly go or to not sing. The main argument seems to have been based on the fact that Latin and German and other familiar European languages cannot split their infinitives. But at least nowadays it seems just silly to try to force one language into the patterns of another.
It ain't about good and bad language; it's about class and education
If the standards come instead from among the speakers of the modern language, the question is which speakers. Language usage varies both with region and with social group, so if we want to standardize, we need to choose a region or group. As noted above, there are sometimes political considerations involved. In many countries, standard usage comes from the speech and writing of well-educated or upper-class people from a particular region, though the regional origin of the standard is not so clear in the case of the United States. For example, the current English standard prefers you aren't to you ain't because the latter form, although common in regional dialects all over the English-speaking world, is associated with uneducated speakers.
Actually this argument only holds today. The history is more complicated. Ain't was once used by upper-class speakers, but prescriptive grammarians who felt that it was somehow "lazy" or "illogical" succeeded in mostly obliterating it from upper-class speech in the English-speaking world. See this short history of ain't from the American Heritage Dictionary of the English Language (2000).
Another example is the pronunciation of pairs of words like pin/pen and since/sense as the same. This is a feature of the speech of a large region of the United States, including the South and some areas bordering on it, but it would not be considered a feature of standard American English.
How standards are enforced
Trying to teach grammar to native speakers
Although their efforts rarely seem to have an effect on children's speaking, in a limited way they may affect the children's written language. This is at least possible when the "rule" amounts to the prohibition of a particular form, for example, ain't. When it is more complicated, the prescribing may backfire, leading to behavior that is not what the prescribers had in mind. An interesting example is the use of "subjective" pronouns (I, he, we, etc.) vs. "objective" pronouns (me, him, us, etc.) in English. In an attempt to prevent usages such as him and me are friends, teachers have created a situation in which many speakers now say with he and I, instead of with him and me, not at all in accordance with the original intention of the teachers. This is an example of hypercorrection, which occurs when a prescriptive rule is applied in too many cases. For more on the issue of hypercorrection with English pronouns, see this interview with English professor Jack Lynch, which explains the usage but also takes a prescriptivist approach.
In summary, we've seen two sorts of usages that people try to enforce, usually attempting to prevent an "incorrect" form. In one situation, there is a form perceived as incorrect, such as ain't, that is characteristic of speakers of some dialects but not normally part of the speech or writing of speakers of the standard dialect. In another sort of situation, there is a form perceived as incorrect, such as "singular they", which is used by many or most of the speakers of the standard dialect as well as by speakers of other dialects. In this second sort of situation, there is often disagreement about what should count as the standard, and it is this kind of case where linguists sometimes become involved because of their interest and expertise in what people actually say or write. (My advice is not to get involved in one of these arguments. For some reason it often seems hard for people, at least for English speakers, to be rational about what counts as "correct" or "incorrect".)
I have focused on English. For a brief history of English "usage", that is, the concern for what is right and what is wrong in standard English, see this passage by E. Ward Gilman from the 1989 edition of Webster's Dictionary of English Usage. Not surprisingly, there are similar discussions and debates concerning usage in some other languages, though this sort of concern does not appear to be at all universal.
Other evaluation of language
So far we have looked at attempts to enforce linguistic standards on people. But people, especially composition and creative writing teachers, are also involved in evaluations of language based on other kinds of criteria. One is appropriateness, discussed in the last section. Appropriateness is one aspect of language that we continue to learn as adolescents and adults, and it is often explicitly taught. For example, a student might be corrected for using a word or phrase perceived as too casual in certain contexts, say, be into in the sense of 'be interested in' in an essay. Or an employee might be corrected by a colleague for calling his boss "dude".
Another important kind of evaluation concerns the effectiveness of language. A usage may be grammatical and appropriate to the context but still not accomplish the speaker's or writer's goal. For example, language may be needlessly ambiguous, as in the following example.
A composition teacher might criticize the student's use of it in this sentence; does it refer to the coconut or the screwdriver? Or a use of language may fail to accomplish a higher-level goal of the speaker or writer. Consider the following example at the beginning of an argument.
A teacher might find the writer's point trivial and obvious and suggest leaving it out.
These are all legitimate reasons for evaluating language of course. While linguists and other language scientists are not in the business of evaluating people's language, they can be of help by studying what appropriateness is, what makes an expression interpretable by a hearer or reader, and how the parts of a text relate to one another.