M.A. / The literature is unclear whether bilingualism has cognitive advantages, disadvantages or neutral cognitive effects on the child. In order to investigate this discrepancy in the research data, this study aimed at comparing the nonverbal cognitive abilities of a fairly large sample monolingual versus bilingual girls, while comparing for certain variables, not addressed in the literature. The research question is thus formulated as follows: Are there differences in the nonverbal cognitive abilities of monolingual versus bilingual girls, when measured with four non-verbal cognitive test batteries? According to the literature, first language development can be divided into pre-linguistic and linguistic speech, while second language development is either acquired simultaneously or sequentially. Many theories attempt to explain how first and second languages are acquired and used. These theories on the developmental process of first and second languages can be divided into learning, nativist and cognitive theories. Learning theories focus on the effect the environment has on language acquisition, while nativist theories accentuate the influence of genetic factors. Cognitive theories focus on the child's cognitive development as a prerequisite for language acquisition and functioning. Bilingualism is defined differentially in the literature, which presents one of the major reasons for divergent results. In this study a person was defined as bilingual when he or she was capable of one language function, such as reading, writing, listening or speaking at any level of proficiency. Various factors may, however, influence a person's bilingual status. In this regard, age and method of acquisition is important, as well as patterns of sage of the second language, socio-emotional factors and the linguistic relationship between the two languages.