Paul outlines this a bit in Romans 5 in constrasting the role of Adam as representative vs. Christ as representative (emphasis mine).
12Therefore, just as sin came into the world through one man, and
death through sin, and so death spread to all men because all sinned—
13for sin indeed was in the world before the law was given, but sin is
not counted where there is no law. 14Yet death reigned from Adam to
Moses, even over those whose sinning was not like the transgression of
Adam, who was a type of the one who was to come.
15But the free gift is not like the trespass. For if many died
through one man’s trespass, much more have the grace of God and the
free gift by the grace of that one man Jesus Christ abounded for many.
16And the free gift is not like the result of that one man’s sin. For
the judgment following one trespass brought condemnation, but the free
gift following many trespasses brought justification. 17For if,
because of one man’s trespass, death reigned through that one man,
much more will those who receive the abundance of grace and the free
gift of righteousness reign in life through the one man Jesus Christ.
18Therefore, as one trespass led to condemnation for all men, so one
act of righteousness leads to justification and life for all men.
19For as by the one man’s disobedience the many were made sinners, so
by the one man’s obedience the many will be made righteous. 20Now
the law came in to increase the trespass, but where sin increased,
grace abounded all the more, 21so that, as sin reigned in death, grace
also might reign through righteousness leading to eternal life through
Jesus Christ our Lord.
As far as fairness goes, the short answer, I think, is that it's just as fair counting (presumably, sinless?) men as sinners under the representation of Adam as is it counting (presumably, sinful) men righteous under the represenation of Christ :) For a long answer, I think there's a bit of a clever twist in there, in that Adam's representation seems to be the very embodiment of the attitude that God's design is not just or fair and the presumption that man knows of a better way...IOW, the irony is that even if we weren't represented collectively by Adam; the representation he represents actually seems pretty fair and accurate.
The even longer answer is related to covenental therology which includes the notion of "federal headship" (FH). I think an important key to the idea of FH is a distinction between an intrinsic "is" and a forensic (or legal) one. The FH view is that humanity is forensically linked to Adam (note that the curse in Genesis was applied to all of creation; not just him),
** As a bit of an aside, doctrine of "original sin" deals more with intrinsic inheritence whereas "federal headship" deals more with forensic inheritence....it's a somewhat subtle difference, and many folks hold to both, but it's not logically necessary to hold to (or reject) them in tandem.
Convental theologians would argue that the theme of covenent and federal headship permeates the entire narrative of Scripture: Adam, Noah, Abraham, Moses, priests, kings (esp. David), Christ...and maybe even more generally husbands/fathers...all of these were, in some ways identified as particular representative "heads" of a larger community. As the head of the community rose/fell, so, too, did the community itself.
I think the clearest example for Christians is probably found in an analysis of the double imputation of Christ--our (intrinsic) sins were imputed (forensically) to him, and his (intrinsic) obedience is imputed (forensically) to us.
Christ was intrinsically righteous (he was perfectly obedient), and through his headship, his people are declared (forensically) righteous. Even though we're not obedient by our own right, we are deemed as such by virtue of being found "in Christ." It's important to note that our justification is not found in our personal obedience or works (lest any man should boast); the mechanism for this is the federal headship of Christ--i.e. he's our representative; we are his people.
Likewise, our sins are still atoned through that same mechanism, but in reverse; Christ is not intrinsically sinful ("He who knew no sin..."), but by being our representative, he is forensically cursed through the sins of his people ("...became sin"). In a very real sense the sins of the redeemed have carried with them the penalty of death in the same way the sins of the condemened have...it's just that Jesus--as head of the redeemed/as our represenative-- paid the price of his people. We are his sinful people; he's our represenative.
Adam, as a type of Christ played a similar role. As first man, he was a representative who specifically and instrinsically broke the original commandment. Through his disobedience, we're all (forensically) condemned.
A modern day parallel would be something like a diplomat/ambassador and/or national leader, who, though he is an individual, speaks on behalf of his nation. The corporate nation is bound to the promises and treaties that its leaders make even if, as individuals, we would never make some of those promises to begin with.
I found this link which explains this "federal headship" in relation to covenant theology. It's an interesting read, and summarizes thusly:
The Importance of Covenant
Covenant is the fabric of the whole Bible. Once this fundamental
schema of covenant in the Scriptures comes clear, all the patterns of
God's relations with the sons and daughters of Adam unfolds into a
rich tapestry unifying the Scriptures.
We have seen that Adam in Romans 5:12-21 was the federal
representative of his race under the covenant of works. Some
theologians reject this understanding of Paul's teaching outright,
because it "violates all sense of justice."20 But if we are to use our
"sense of justice" as an ultimate criterion for judging the truths of
Scripture, then shouldn't we deny all covenant imputation as well? If
sin cannot be imputed from one to many, conversely it cannot be
imputed from many to one. Under this method, how can we maintain that
"He himself bore our sins in his body on the tree" "the righteous for
the unrighteous" (1 Pet. 2:24; 3:18; cf. Isa. 53)? Shouldn't this
violate our sense of justice, too? And if our sins were not imputed to
Christ, neither can his righteousness become ours (e.g., 1 Cor. 1:30;
2 Cor. 5:21). Then we would all be cut off from Christ and personally
obligated (as was Adam), to keep all of God's holy law ourselves (Gal.
In contrast to this grim prospect, Covenant theology offers a
fresh restatement of classic Protestant insights into the essential
truths of justification as the imputed righteousness of Christ by
grace alone and received by faith alone. What makes imputation work is
covenant, for covenant is the forensic instrument by which God
faithfully extends his blessings to the heirs of the covenant of
grace. The curse on Adam was not the last word on covenant in the
Bible. This is what excites Paul in Romans 5:12-21 and what excites
covenant theologians as well:
God's grace and the gift that came by the grace of the one man, Jesus
Christ, overflow to the many (v. 15) ... the gift followed many
trespasses and brought justification (v. 16) ... those who receive
God's abundant provision of grace and of the gift of righteousness
reign in life through the one man, Jesus Christ (v. 17) ... the result
of one act of righteousness was justification that brings life for all
men (v. 18) ... through the obedience of the one man the many will be
made righteous (v. 19) ... grace might reign through righteousness to
bring eternal life through Jesus Christ our Lord (v. 21; NIV).