This chapter follows current terminology and use the term 'learning difficulties' rather than subnormality, when describing research that used the older terminology. IQ tests are central to our current definitions of learning difficulties. The tests are standardized, by testing large and representative samples of people of different ages, to obtain norms for performance. Another measurement, related to IQ, that is often used to express the level of intellectual development is mental age (MA). Mental age is given simply by the level of attainment reached on a test in relation to the normal population. In the Education Act, a distinction was made between Educationally Subnormal (ESN) mild and severe. Prenatal maternal infections, such as rubella and syphilis, can adversely affect the brain of developing foetus. The Down's chromosome anomaly results in a deceleration of all aspects of development. It uses a combination of cross-sectional and longitudinal studies to infer developmental changes in memory processes in studies reported in later chapters.