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Three studies that used experimental manipulations of stimulus context and correlational analyses were conducted to examine how contextual effects influence magnitude estimation and the crossover effect on line bisection. Previous work had shown that although orienting attention to one end of a line prior to bisection determines the direction in which crossover occurs, bias in magnitude estimation actually produces the crossover effect. The influence of contextual effects on magnitude estimation, however, was not examined in these previous models of crossover. Consequently, the purpose of the present investigation was to examine these effects. Subjects in the current studies were healthy controls and people who had right and left hemisphere injury due to stroke, both with and without spatial neglect. Study 1 examined the crossover effect for lines bisected with and without a stimulus context. Study 2 examined both stimulus order as well as response order context effects on magnitude estimation. Study 3 examined how much variance in magnitude estimation was accounted for by stimulus contextual effects and how stimulus context influenced the crossover effect. The results showed that contextual bias was ubiquitous but relatively small in the magnitude estimates of normal subjects. Contextual bias was exaggerated to a similar degree in subjects with right or left hemisphere injury due to stroke, but the amount of variance accounted by contextual bias was still quite small. A novel finding of study 2 was that contextual effects can be induced by previous responses to stimuli as well as by the magnitude of preceding stimuli in subjects with unilateral brain injury. This may be a contextual effect related to response perseveration. Finally, studies 1 and 3 indicated that contextual effects strengthened the crossover effect on line bisection, primarily on relatively short lines. Contextual effects, however, cannot fully account for the crossover effect, because crossover bisections were observed also in the absence of a stimulus context. It is concluded that the crossover effect is explained by biases in attentional orientation and magnitude estimation. Contextual effects represent one source of bias in magnitude estimation that influences the crossover effect by promoting contralateral errors on short line lengths (<2 cm)., Competing Interests: Conflicts of interest The authors have no conflicts of interest to declare.