In a 1989 international test of 13-year-olds, Koreans finished first in mathematics and Americans finished last.
Yet when asked whether they thought they were "good at mathematics," only 23 percent of Koreans said "yes," compared to 68 percent of Americans.
Self-serving bias is the tendency to overrate oneself--to be overly confident of one's knowledge, abilities, or good fortune.
The American students in this study certainly don't lack "self-esteem." What they do lack is a proper sense of how little they know about mathematics.