Mathematics has always been used as a tool for organizing and understanding the physical sciences. Today mathematics is also applied to other disciplines such as biology, medicine, management, linguistics, and the social sciences.
Initially, the area of mathematics that was of primary importance outside the physical sciences was classical statistics, used in the collection and analysis of data. More recently there has been a growing interest in the construction of non-statistical models. These endeavors coexist and sometimes overlap, but are considered to be different aspects of their respective disciplines. Some examples: Economists who specialize in the application of statistics to their field are called econometricians; biologists, biometricians; psychologists, psychometricians, and so on. Economists who are primarily interested in non-statistical modeling are called mathematical economists. Similarly there are mathematical biologists, mathematical psychologists, and so on. The growing importance of these professions provides an opportunity to combine mathematical training with a serious interest in another discipline.
What should one study in college?
There is no well-defined educational path for students wishing to enter these interdisciplinary areas, although it can be safely said that there is little opportunity without a doctoral degree. A strong undergraduate mathematical education with a double major would be the ideal start. Short of that ambitious program, a major in mathematics with considerable course work in the other field would be a good beginning.