Many of the decisions we are called upon to weigh up have both advantages and drawbacks. The fallacy of one-sided assessment is fallen into when only one side of the case is taken into consideration. Decisions usually require both pros and cons to be taken account of, and a preference made for the side that wins on balance. To look at one side only is to avoid judgement of that balance:
I’m not going to get married. There would be all that extra responsibility not to mention the loss of my freedom. Think of the costs of raising children and putting them through college. Then there are the inc insurance premiums…
(If that really were all, no one would ever do it.)
It is equally possible to look only at the positive side.
This encyclopedia is one you will be proud to own. Your friends admire it. Your children will benefit. You will learn from it. It will complement your bookshelf!
(On the other hand, it will cost you a LOT of money.)
Either way, the fallacy of one-sided assessment is committed. By looking only at the objections, or only at the advantages, we are excluding material which bears on the decision, and which should be taken account of. The omission of this relevant material from the argument is what is fallacious about one-sided assessment.
One-sided assessment is not a fallacy when space is allocated for the other side to be presented similarly. There is in Anglo-American culture an adversarial tradition, which has it that if each side has the strongest case put forward, then a dispassionate observer is likely to arrive at a fair judgement. We therefore expect a counsel to put only the evidence for acquittal, and a trade union negotiator to put only the case for an increase, because we know that there will be someone else putting the other side. It would be one-sided assessment if those making the judgement considered one side only.
Let’s not go to Ibiza. Think of the heat, the mosquitoes and the crowds.
(On the other hand, what about the lovely sunshine, the cheap wine, the excellent food and the low prices?)
Life’s judgements often call for trade-offs. Those who have made their balance and come down in favour are apt to try to persuade others by emphasizing only the positive side. The unwary should remember that their own scale of values might call for a different judgement, once they consider all of the factors.
All of the arguments support the new road. It means progress; it means prosperity; it means a future for our town!
(And it really is rather unfortunate that they have to pull your house down to build it.)
There is a clever version of one-sided assessment which you should use when persuading others to agree with your judgment. This involves making a purely token concession to the case against you, by referring to one of the weaker arguments on the other side before you launch into the overwhelming arguments in favour. This polishes your case by adding to it the gloss of apparent objectivity.
Of course, if we bought a bigger car, we’d need to make new seat covers. But think of the convenience! All the shopping would go in the back; we could use it for holidays; you could pick up the children in comfort; and its extra speed would cut down our journey times.
(Sold, to the gentleman with the fallacy.)