Thumb On The Scale Meaning

(idiomatic) An act of bias or a tactic for cheating which creates a situation that unfairly benefits one party involved in an interaction.

Example: 1982, Emory A. Griffin, Getting Together, ISBN 9780877843900, p. 68 (Google preview):
  I'll try to give an impartial account without putting my thumb on the scale.
1991, Deborah L. Rhode, Justice and Gender, ISBN 9780674491014, p. 187 (Google preview):
  [E]ven to characterize these cases as affirmative action is misleading. What critics often present as a thumb on the scale for "less qualified" individuals may in fact reflect only necessary adjustments in the way the scale is calibrated.
2009 June 23, "China and US head for trade war," Guardian (UK) (retrieved 1 Nov 2015):
  Ron Kirk, the US trade representative, accused Beijing of putting a "giant thumb on the scale" by restricting exports of commodities including silicon, coke and zinc, to give Chinese manufacturers an unfair advantage.
2014 Oct. 22, Derek Willis, "‘Soft on Crime’ TV Ads Affect Judges’ Decisions, Not Just Elections," New York Times (retrieved 1 Nov 2015):
  "The data show that the television campaign ads this money buys put a thumb on the scale in criminal cases, and undermine the promise of equal justice."