Social Desirability Bias
Social Desirability Bias is a cognitive bias (a general pattern or tendency to think a certain way) in which people respond to questioning in ways that make them seem more favorable or appealing to others. In this case, people over-report their positive behaviors or qualities while under-reporting undesirable or or negative behaviors and qualities.
When conducting research, this can cause many problems and lead to incorrect and biased results. It also has many negative effects in every day life. For example if you don't tell your doctor about habits that may appear negative, such as eating a lot of junk food or smoking, the doctor will not have the right information to treat you properly, and could even lead to false conclusions, unnecessary testing, and improper diagnoses.