The linguistic relativity hypothesis has focused on the influence of grammar in language on speakers' cognition. Previous studies show that speakers of languages with grammatical number (e.g., English) are more aware of the number of objects. Additionally, recent studies reveal that bilinguals who speak languages with different grammatical structures may alter their cognitive preferences based on their level of language proficiency. This research builds on these findings, following Lucy's approach (1992a), to compare the cognition of monolingual speakers among members of a group consisting of thirty native Thai speakers and thirty native English speakers, with that of bilinguals (Thai-English) with varying proficiency levels in a group consisting of thirty basic-level Thai-English bilinguals, thirty intermediate-level Thai-English bilinguals and thirty advanced-level Thai-English bilinguals. Attention tests and memory tests were implemented to test the level of cognition of each participant, the results being analyzed using ANOVA and Scheffe's test. The results showed that the English-speaking subjects paid more attention to the number of objects and memorized a greater number of them than the Thai-speaking subjects, suggesting that the presence of grammatical number in English and its absence in Thai played a significant role. The advanced-level Thai-English bilinguals paid more attention to the number of objects and memorized a greater number of them than the intermediate-level Thai-English bilinguals, the basic-level Thai-English bilinguals, and the monolingual Thai speakers, respectively, but paid less attention than the monolingual English speakers. However, there was no significant difference between the basic-level Thai-English bilinguals and the monolingual Thai speakers. This finding may lead us to conclude that: 1) grammatical representation affects speakers' cognition, supporting the linguistic relativity hypothesis; and that 2) bilingualism affects cognition at different levels. This finding suggests the possibility that acquisition of a second language affects bilinguals' cognition, and highlights the importance of promoting bilingualism in Thailand in order to enhance English proficiency and global competitiveness. By highlighting how grammatical differences affect attention and memory, the study suggests language education policies should focus on bilingualism and address cognitive impacts, helping Thai learners overcome linguistic challenges and enhance cognitive and language skills.