The theory of minimal brain dysfunction 

The observation that defined brain damage was associated with psychiatric disorder led to the suggestion that lesser degrees of damage, insufficient
to cause definite neurological signs, could account for otherwise unexplained disorders. The term minimal brain damage was suggested, but was later changed to minimal brain dysfunction after repeated failures to find evidence of any structural changes. 

It was suggested that this brain dysfunction originated in damage at birth and for a time the phrase ‘a continuum of reproductive casualty’ (Pasamanick and Knobloch 1966) was used to express this notion. There is an association between histories of abnormal pregnancy, prematurity, and birth asphyxia on the one hand, and psychiatric disorder on the other, but the former factors are also associated with social disadvantage which could be the real cause of the psychiatric disorder. Whilst the concept of minimal brain damage (or dysfunction) has been abandoned, the role of established brain damage is well recognized.