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Would have implies they didn't use lead buy if they had it would have lowered IQs

May have lowered IQs based on extrapolation and modern studies

Might be a better way to put it

Both are valid ways of using "would have", in this case I'm using it as the past tense of "will have". But I appreciate the ambiguity it creates.

> We use "would have" as the past tense form of will have:

>> I phoned at six o'clock. I knew he would have got home by then.

Note the difference between this and:

> I phoned at six o'clock. I knew he was home by then.

Which implies first-hand knowledge of his location. The first only signals a logical conclusion.