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Since most older residents are no longer involved in the workforce and do not derive direct economic benefits from tourism, they are not in a position to trade off negative impacts against pecuniary gains in the way postulated by social exchange theory.

(1)trade off........against

or

impacts against?

(2)What's the meaning of the highlighted sentence?

Thank you in advance!!Emotion: surprise
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Trade off means you accept something bad in order to have something good. You trade off negative impacts against positive impacts.

In the above sentence the trade off is between negative impacts (not specified, but I would assume something like over-crowding, increase in crime, or commercialization of natural habitats, etc.) and positive effects (pecuniary or monetary gains).

Hope this helps.
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To Danyoo:

Thanks a lot!