PROMOTING POTENTIAL 291remove obstacles to it, and guide intervention, as necessary. Of course,
our models of human development are still unclear and contentious. I
have explained this as partly due to vagueness about what develops and
how and also due to the infl uence of ideological preconceptions, espe-
cially as they pervade the nature- nurture debate.
In this book, I have been contrasting more recent dynamical models
with traditional mechanical (input → output) models. Th e fi rst thing I do
in this chapter is illustrate how the mechanistic input → output model has
dominated thinking about both “ge ne tic” and “environmental” causes of
individual diff erences and has shaped interventions accordingly. I then
criticize these and discuss alternative, dynamical, perspectives. In chapter 11,
I apply the analy sis to the institution specially set up to promote the
development of potential, namely, education.
GE NE TIC INTERVENTIONSUntil recently, nearly all conceived ge ne tic intervention in human develop-
ment has been medical, dealing with single- gene conditions or disorders.
Th ese are associated with ge ne tic mutations that either arise in the life of
an individual or can be inherited from parents to off spring. Th ere are
thousands of known single- gene (or monogenic) disorders, occurring in
about one in every hundred births, and many of them have distinctive
eff ects on cognitive functioning.
Research into pos si ble treatments of such disorders has been con-
ducted almost since genes were discovered, in some cases with im-
pressive results. Because of their basis in single genes, and categorical
consequences, intervention has been relatively straightforward and un-
controversial, once cause and eff ect is understood. Th e standard exam-
ple is that of phenylketonuria, an enzyme defi ciency that, untreated, can
lead to intellectual disability, seizures, and other medical prob lems. An
understanding of its ge ne tic basis—or, more accurately, the role of the
gene product in metabolism— led to the dietary intervention that pre-
vents the development of the disease.
Such environmental interventions with these inborn errors of me-
tabolism are uncontroversial. Some excitement has been created by the
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