In 2010-11, 56% of US public school teachers had a master’s or higher degree, while only 43% of US private school teachers had a master’s or higher degree (43 percent) (source).
In 2014, only 13% of Singapore school teachers had a master's or higher degree (4233 Master's + 104 PhD, out of 32,779 teachers) (source: p. 15 of this document).
Based on these 3 data points and my possibly-erroneous presumption that Singapore schools > US private schools > US public schools, it would seem that there is a negative correlation between teaching effectiveness and whether teachers have master's or PhD degrees.
My question is: What does the current research actually say? Is there a positive, negative, or no correlation between a teacher's academic qualifications and his/her teaching effectiveness?