This study aims to first investigate the effect of the extreme response style (ERS) which could lead to an attitude-achievement paradox among the countries participating in the Trends in International Mathematics and Science Study (TIMSS 2015), and then to determine the individual- and country-level relationships between attitude and achievement by adjusting the effect of ERS. For the sample of this correlational study, 500 students were randomly selected from each of the 15 countries that participated in TIMSS 2015. The differences in the ERS tendency of the countries were determined by performing MANOVA. To determine the effect of ERS, two different multidimensional item response theory (MIRT) models were used: one did not include the ERS trait as a dimension while the other included this trait as a dimension. The results were analyzed with Latent GOLD 5.1 and WinBUGS software. To determine the relationship between attitudinal variables and achievement, the correlation values based on the observed scores and MIRT models were obtained. Whether there was any significant difference between these correlation values was determined by Fisher's rz transformation. The findings of this study were as follows: (a) the model in which the ERS trait was included as a dimension best fit the data and (b) the correlation values based on the observed scores were negative and those based on the MIRT models were positive, with the two statistically differing from each other. ERS is one of the factors causing the achievement-attitude paradox; however, it not sufficient to explain this paradox.