Speech samples were taken from 21 children aged 16-40 months covering a wide range of mean utterance length. Presence or absence of 14 grammatical morphemes in linguistic and nonlinguistic obligatory contexts was scored. Order of acquisition of the morphemes was determined using two different criteria. The rank-orderings obtained correlated very highly with a previously determined order of acquisition for three children studied longitudinally. Age did not add to the predictiveness of mean length of utterance alone for grammatical development in terms of which morphemes were correctly used. The approximately invariant order of acquisition for the fourteen morphemes is discussed in terms of three possible determinants of this order. Frequency of use in parental speech showed no correlation with order of acquisition, but grammatical and semantic complexity both correlated highly with acquisition order.

Approximately 40 experiments on rats, monkeys, pigeons, and in one case, human beings have obtained functional relations between some parameter of reinforcement and some measure of response strength. With few exceptions, the obtained functions fit the same mathematical form: Strength of responding appears to be proportional to its relative reinforcement. This principle is, furthermore, logically consistent with the matching law for simple choice, according to which the relative frequency of each alternative in a choice matches its relative frequency of reinforcement.Confronted with choices differing only in the frequency of reinforcement, subjects match the distribution of response alternatives to the distribution of reinforcements (de Villiers, in press;Herrnstein, 1970Herrnstein, , 1971. That is to say, the choices come to obey the following equation, in which RI to R n enumerates the response alternatives and r x to r n the reinforcements associated with each:Equation 1, sometimes called the matching law, says that the relative frequency of each kind of responding equals, or matches, the relative frequency of its reinforcement. 1 A large experimental literature supports the matching law. Although some workers have proposed other formal representations (Baum,

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Various arguments are reviewed about the claim that language development is critically connected to the development of theory of mind. The different theories of how language could help in this process of development are explored. A brief account is provided of the controversy over the capacities of infants to read others' false beliefs. Then the empirical literature on the steps in theory of mind development is summarized, considering studies on both typically developing and various language-delayed children. Suggestions are made for intervention by speech language pathologists to enhance the child's access to understanding the minds of others.