Studies that attempt to clarify and test the Whorfian hypothesis, which states that the structure of one's language influences understanding of reality and behavior, are summarized. Both first language and second language behaviors were investigated in order to determine interactions between language behavior and personality development. Ability to produce native-like pronunciation, degree of gender marking, and devices for segmenting the past were studied among several age and language groups. The cumulative results indicate a complex pattern of interaction between personality and language. Language appears to have mostly an accelerating effect on the growth process but does not create permanent differences. The influence of personality on language behavior is more pervasive and enduring than the influence of language on personality. (RW)