To say that race is a social construction means that it and its definition grow out of the process of human interaction. This means that race is what interacting humans define it to be. In this sense, how you are perceived in a community of peers in part defines your race: If your friends and associates see you as African American, then in that one respect you are indeed African American. As another example of social construction, if race-like divisions among people in this society were made only on the basis of who had red hair and who did not, then people would come to think of races as being defined by hair color—a physical characteristic. Over time, this definition would come to be upheld by society's institutions—by the courts, by the educational system, by the federal government, and so on. It might well come to be thought of as a truly "scientific" classification, since it is based on a physical 'characteristic (hair color instead of skin color), and as everyone knows, redheads are different from everyone else—they have fiery tempers and argue a lot, don't they? Thus, racial stereotypes would soon be applied to redheads! Social construction means that people learn, through socialization and interaction processes, to attribute certain characteristics to people who are classified into a racial category. These are just what racial stereotypes are: attributions that are for the most part not true and yet stubbornly persist over time. These stereotypes are social constructions. They are generally based on only a small truth (if on any truth at all) and are then thought of in society as applying to all members, and to "typical" members, of some racial category. Stereotypes are generally negative. We have all heard the common stereotypes: Blacks are inherently musical, possess "natural" rhythm, are loud, and crime-prone; Asians are sneaky and overly conforming; Hispanics are naturally violence-prone and carry knives; American Indians are quiet, subservient, and underachievers. These traits are seen by society as inherent to any member of the particular group. In fact they are seen as essential to the group identified by the stereotype. Sociologists call this process essentialization: Such negative stereotypical traits are regarded by society as essential (inherent) to the character of any person identified by the stereotype. Negative stereotypes, thus negative essentializations, are applied far more to minorities than to Whites, and they thus help define what it means to be minority in society, even though Whites are sometimes ridiculed as having a few negatively stereotyped traits ("White boys can't jump," "blondes are dumb," and so forth).

SOURCE: Higginbotham, E., & Andersen, M.L. (2006).  Race and Ethnicity in Society: The Changing Landscape. Instructor’s Edition. Belmont, CA: Thompson Higher Education. Page 49-50.