Although the change in the development runs parallel with the chronological age, age in itself cannot cause the development. The basic mechanisms or the causes of the evolutionary change are the genetic factors and environmental factors. The genetic factors are responsible for the cellular changes like the global growth, the changes in the proportion of the body and parts of the brain, and the dietetic maturation of the aspects of the function like the vision and necessities. Because the genes can “be extinguished” and “ignited”, the initial genotype of the person can change with time in function, giving rise to changes in the later development. The environmental factors that affect the development can include the diet and the exhibition to diseases, as well as social, emotional, cognitive and experiences. Nevertheless, the examination of the environmental factors also sample that the young people the human beings can survive within a quite ample rank of environmental experiences.

Instead of acting like independent mechanisms, the genetic and environmental factors often interact to cause the evolutionary change. Some aspects of the development of the boy stand out by their plasticity, or the measurement in which the direction of the development is guided by factors environmental, as well as initiated by genetic factors. For example, the development of allergic reactions relatively early seems to be caused by the exhibition to certain environmental factors in the life, and the protection against the early exhibition does that the boy less prone to the allergic reactions later. When an aspect of the development is affected by the early experience strongly, it says that they show a high degree of plasticity, when the genetics is the main cause of the development, the plasticity says that is low. Plasticity can imply a direction according to the endogenous factors, like hormones, as well as by exogenous factors like the infection.