Suppose people notice that crimes are getting less frequent but more violent; or that Thai restaurants are replacing Chinese restaurants at the bottom end in Chicago; or that young people are using instant messages more than telephone or email. Are these social trends? I suppose that, most simply, these are statements about changing frequencies of certain kinds of social occurrences over time. And "noticing" means counting and tracking the results of past observations. There are important conceptual and measurement issues here -- how we identify the kinds of events whose frequency is "important" or "of interest"? How do we assure that we are counting in a way that is accurate -- not over- or under-reporting? But the basic logic is fairly clear. To say that there is a trend for X is to say that the frequency of X relative to the population is changing in a sustained way.
Is the discovery of social trends an important effort for sociology? Probaby so, for several reasons. We are interested in knowing "how society is changing" -- and the frequencies of various kinds of social actions are themselves an important component of social change. So discovering and documenting changing patterns of social behavior -- "trend-spotting" -- is an important piece of sociological discovery.
But there are two other reasons to think trends are important for sociology. A trend (more Thai restaurants) may be an indicator of some other more important social change -- aging of the Chinese population in Chicago, or an influx of Southeast Asian immigrants in a certain time period. But, second, some social trends may function as causes of other future changes in social behavior. A rising trend in male-female sex ratios in China or India may be a potential cause of future social disorder; A rising frequency of soccer violence may be both an indication of rising youth alienation and a cause of future state action (more repression, more social welfare intervention). A trend towards longer sentences for non-violent crime may produce a rise in violent crime in the future (as non-violent criminals "graduate" to violent crime through longer exposure to violent criminals). Finally, discovery of trends can produce strategies of adaptive behavior -- trends in consumer taste may permit some businesses to create a whole new market for a product, trends in violent crime may produce new policing strategies, and trends in young voters using the web for communication may suggest new campaign tactics.
Finally, it should be noted that some "trends" may not be true features of social behavior at all, but rather the reslt of heightened awareness of certain kinds of social behavior. (Teachers often say "our students are getting worse every year" ... even though the statistics on performance say the opposite.)
We asked above what is involved in explaining social trends. Surely there must be a range of different social mechanisms that would produce change in the frequencies of various social behaviors: incentives, filters, external shocks, imitation, changes in the composition of the underlying population. For example, suppose we observe a trend towards fewer incidents of purse-snatching in Miami, and we observe as well that the median age of the population has
increased from 50 to 60. The change in the age structure may explain the trend. In this case, there may be no change at all in the behaviors of the various age groups -- no trend there -- but a change in frequency relative to the total population nonetheless.
(This discussion of "social trends" places the concept in the domain of "changing distributions of individual social behavior". Is there a structural counterpart about collective entities? Can we make true and justified statements about the trends of change among -- labor unions, states, churches, or universities?)