Are programmers usually included as co-authors in psychology? I work as a programmer supporting a psychology lab. 

Most of my work is translating some paper-based or physical assessment into a computer-based equivalent or I create programs for novel assessments along with associated databases and support programs for data retrieval and basic cleaning. 

Recently, a colleague included me as a co-author on a paper.

Are they are being overly nice or should I be a co-author on other papers?
In general, only researchers are included as authors. In some cases, the person doing the programming makes a critical intellectual contribution and becomes a researcher. In Psychology, specialist help is often brought in for programming, statistics, modelling, animal care, data acquisition, drug administration, and subject treatment. These support staff are often just turning a "crank". The crank is not necessarily easy to turn and the project would not be completed without it being turned, but turning the crank is not research.

Support staff tend to be mentioned in the acknowledgements. Programmers tend to get the short end of the stick in that they get acknowledge only the first time the software is used while other support staff get mention on every paper they are involved with. Support staff only become authors if they do something novel (for example, develop novel testing software). In these cases, they would be an author on the paper (often a methods paper) describing that novel contribution.