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I have seen two different uses of "Who's to say X". It appears to me, that the author could either defend X in a sarcastic or ironic way, or could attack X by presenting evidence contrary to X. My question is: is my understanding correct in that "Who's to say X" could be used in two opposite ways?

Example 1: Who’s to say that we’re to blame for global warming? Source 1

In my understanding, example 1 means: I challenge you to prove we're to blame for global warming. We're probably not to blame for global warming. That is, there are external factors or circumstances outside our control which cause or at least contribute to global warming.

Example 2: Medicare is portrayed as getting the best deal from the system because Medicare pays less per service. But remember how the system works. Who's to say Medicare doesn't pay less per procedure because it's being billed for many more procedures, because that's how providers are allowed to maximize their revenues from the payer known as Medicare? In fact, plenty of evidence suggests this is exactly how Medicare operates. Source 2

In my understanding, example 2 means: Medicare is commonly portrayed as less costly. I challenge you to prove it costs more according to this complex and not frequently-analyzed approach. Well, in fact, it does cost more! And exactly due to the aforementioned, not well-known technique!

Is my understanding of the usage a correct one? Particularly, in example 2, I feel that my interpretation could be improved.

Thanks for any help.

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Who's to say? is not the same as Who says? – TimLymington Jun 19 '13 at 20:30
I can't really support this (hence the comment) but I feel that who's to say should always be followed by a negation. I mean who's to say John did not want to? rather than who's to say John wanted to?. – terdon Jun 19 '13 at 20:31
@ jrand: You're creating a false dichotomy here. Superficially, your first example is not idiomatic English, since the standard forms are "Who says xxx", and "Who's to say NOT(xxx)". As @terdon says, the latter form always involves negation. The only reason native speakers would use your Ex1 format is when they're stressing it as "Who’s to say that we’re to blame for global warming?" (i.e. - the implied NOT component leads to the meaning: NOT us, but somebody/something else is to blame). – FumbleFingers Jun 20 '13 at 0:45
up vote 1 down vote accepted

The phrase who's to say is a mild approach to disagreeing with a proposition. It suggests that there has not been a thorough analysis of the concept and the speaker is about to point that out or provide proof to the contrary.

As Tim Lymington points out in his comment, it is somewhat different from who says (or colloquially, who sez). This latter phrase is a direct challenge to the advocates of the proposition. A similar rebutal may follow, but it is usually more pointed than the former and may emphasize the bias of the person who says.

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I see the part with who says, and the implied reduction on the credibility of the person who says. If I get your first paragraph right, "who's to say X" could also imply "There's not enough support for X, let's go with not X". But that doesn't seem to explain the example in source 2, where "Who's to say X" is followed immediately by a support of X. – jrand Jun 19 '13 at 20:56
@jrand Actually, the quote is "Who's to say Medicare doesn't pay less per procedure because it's being billed for many more procedures, because that's how providers are allowed to maximize their revenues from the payer known as Medicare." He is saying "Who is to say that Medicare does not pay less because . . ." He then shoots down that proposition, arguing that it does pay less because . . . – bib Jun 19 '13 at 21:57
I see now. Thank you. My current understanding in the sentence and reasoning goes as follows: "Medicare is portrayed as paying less per a single service. Who's to say Medicare pays more (per service)? Let's take the fact into account that more services could be rendered. This is exactly the reason for paying less. To lower the total costs in the equation, Congress enacts legislation which leads to a cut in the price per service". – jrand Jun 19 '13 at 23:51

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