The behavioral sciences have been slow to change partly because the explanatory items often seem to be directly observed and partly because other kinds of explanations have been hard to find.(2002, 62)

explanatory adj. 解释的，说明的

item n.项目，条款，细节，依据 ----- 在此处是依据的意思

行为科学之所以发展缓慢，部分原因是用来解释行为的依据往往可以直接观察到，部分原因是其他的解释方式一直难以找到。