There seems to be a contradiction from what Moses wrote in Genesis and that which he wrote in the Psalms. Gen 6:3 reads:

Then the LORD said, My spirit shall not abide in mortals forever, for they are flesh; their days shall be one hundred twenty years.

This text seems to say that the life span of humans will not be longer than one hundred and twenty years. Yet in Psa 90 we find Moses saying that humankind would only live to be seventy or eighty years of age.

The length of our days is seventy years—or eighty, if we have the strength; yet their span is but trouble and sorrow, for they quickly pass, and we fly away (Psa 90:10).

Does this mean Moses has contradicted himself?

Time before Flood

There are a couple of ways of understanding these passages without seeing them as contradictory. One view has the one hundred twenty years of Gen 6:3 as referring to the amount of years the people would have before God sent the Flood. The subject, therefore, has nothing to do with the life spans of humans.

Not the Upper Limit

Even if Gen 6:3 is dealing with how long humans live, it would not contradict Psa 90. Living to be seventy to eighty years was intended to be the average life span for a human, not the extreme limit. Moses lived to be one hundred and twenty years of age—the upper limit of those who lived after the Flood. Therefore we have two possible solutions to the question—either one will work.