In theoretical accounts of second language acquisition, socioeconomic status (SES) has a great impact on the second language (L2) learning outcomes, where their relationship is suggested to be indirect (Ellis, 1994; Gardner, 1985, 2007). Previous studies mostly focus on the influence of SES on L2 general proficiency or L2 academic achievements rather than specific L2 proficiencies. In the educational context of non-English-speaking East Asian countries, cram schooling is believed to have played a mediating role in the relationship between family SES and English academic achievements (Butler, 2009). The present study clarifies the effects of SES and cram schooling on the L2 written task-based performances of 82 Taiwanese EFL college students by examining the correlations between complexity and accuracy. The high-SES and low-SES groups demonstrated two contrasting outcomes. A negative correlation between grammatical complexity and accuracy (known as the trade-off effect) on L2 written output was detected in the low-SES group. But no trade-off effect was attested among the high-SES EFL learners. Moreover, the cram-schooling factor is suggested to have neutralized the negative correlation in the low-SES EFL learners.