This study investigates the possibility that solitary dialog, in which individuals construct in writing a hypothetical dialogic argument, may more fully reveal individual skill achievement in argument than do conventional argumentive essays. A sample of 54 11-12-year-old Chinese students individually composed such written dialogs, subsequent to their participation in a 4-month dialog-based argument curriculum that previously reported gains in both dialogic and essay assessments. Also partaking in the constructed dialog task reported on here were two non-intervention control groups from the same school; one the same age (n = 50) as and the other 2 years older (n = 52) than the intervention group. As well as outperforming their agemates, the intervention group's performance on the constructed dialog task showed they had achieved skill equal to that of the older group in counterargument and were superior to them in using evidence to justify claims. The possibility is considered that the my-side bias reported in typical argumentive essays is due to limited understanding of the purpose of essay writing, rather than lack of skills per se.