thumb on the scale

[Nefertiti]

"Does income act as 'thumb on the scale'? The evidence analyzed here suggests that it does, and that the disadvantage to lower-income students is more pronounced than some prominent higher education observers claim."

1. What does 'thumb on the scale' mean? Does anyone know its origin?

There's another one: put your thumb on the scales
If you put your thumb on the scales, you try to influence the result of something in your favour.

2. Do 'thumb on the scale' and 'put one's thumb on the scales' have similar meaning?

Thanks

[Tdol]

Yes; it's just a variation to show that it can affect the result. If you put your thumb on the scale when weighing something, it makes it look heavier than it is, so a vendor might do this to cheat someone. Scales are used as a symbol for justice, which is what is referred to in the idiom.

[mfwills]

What does 'thumb on the scale' mean? Does anyone know its origin?

It originates with unscrupulous retailers, most notably butchers (who sold pretty much everything by weight), who might use a thumb to depress the scale platform slightly while weighing a piece of meat, thus charging the customer for more than she was actually getting.

Most butcher shops would have a scale positioned so that the butcher could put something on it to weigh and the customer could see the weight reading, but not what the butcher might be doing with his hands. Thus, it was quite simple to hide this action.

In modern idiom, it refers to the practice of taking advantage of someone by typically covert means, possibly utilizing resources not necessarily wrong in and of themselves, but unknown to others.