Ad hominem ad nauseam

The comments thread lately has been full of what might be called the “”ad hominem fallacy” fallacy”. This is the fallacy that, because a logical syllogism is equally valid or invalid no matter who propounds it, evidence in favour of a judgement about a matter of fact should be treated the same no matter who puts it forward. But classical syllogistic logic has essentially nothing to say in relation to reasoning about the plausibilty of judgements based on evidence.

No one sensible takes this idea seriously when, for example, money is at stake. A member of a board of directors who has a financial interest in a proposal is expected to declare it and withdraw from the discussion for example. By contrast, believers in the “ad hominem fallacy” fallacy would suggest that the director’s arguments were just as valid as anyone else’s, and they do not need to declare their interest before taking part in the discussion (though they should not vote).

The problems with conflict of interest are twofold. First, it is usually impossible to check every factual claim made by someone putting an argument. Second, even if all the facts asserted in support of some position are verifiable, they may have been selected (cherry-picked) to favour a case, while facts pointing the other way have been ignored. If you’re willing to go to the trouble of fully informing yourself about the topic using independent sources evidence from interested sources is redundant, and if not, it’s unreliable.

I had a lengthy go at this here, and for convenience I’ve reposted it over the fold.

There’s more from Don Arthur , Tim Lambert and Cathy Young

A question that’s often raised in relation to public policy issues involving science is whether conflicts of interest matter. For example, does it matter if scientists who publish reports suggesting that the dangers of smoking are overstated turn out to be funded by tobacco companies? Common sense suggests that it matters, but a lot of commentators, often with a vague recollection of classes in elementary logic, suggest that this is an ad hominem criticism and that the only thing that is relevant is the argument, not who makes it. You can see a defence of this position from Elizabeth Whelan at Spiked here[1] (hat tip, Jennifer Marohasy in the comments to this interesitng Catallaxy post on values and science.

I’ll argue that common sense is right here.

As an illustration, suppose you are considering buying a new car, and you come across an “independent non-profit” site called “Car Buyers Guide”, which gives advice on models A and B. Here are some possible reasons the site might advance for buying A rather than B. Assume that you can confirm that all factual claims made are correct, but you don’t know anything about cars yourself.

1. The fuel required for model B is not available in Australia, so it cannot be driven here, unlike A. Therefore you should choose A

2. We consulted ten leading experts. All recommended A

3. We looked at ten different criteria and A was superior on each of them

If you rely exclusively on syllogistic logic you ought to find argument 1 convincing (with the auxiliary premise that a car that can be driven is always better than one that cannot). On the other had, reason 2 is a standard fallacy: an argument from authority. Reason three is also logically invalid; the fact that A is superior on some grounds does not mean that it is superior on most or all grounds.

In practice, though, syllogistic logic is not very helpful. Very few decisions can be supported by watertight logical arguments like 1. In practice, we ought to find reasons like 2 and 3 pretty convincing. Assuming that the 10 experts are selected at random from a suitable population, the probability that most experts actually favour B is less than 1 in 1000. And if the 10 criteria are selected sensibly, it’s highly unlikely that consideration of omitted criteria will change the balance.

You can either accept this kind of reasoning or become an expert on the subject yourself. Since the latter course is feasible in only a few cases, inevitably you have to rely on the former most of the time.

Now suppose you find that the “Car Buyers Guide” is actually funded by the makers of Model A. Reason 1 is still logically valid and compelling. But reasons 2 and 3 now have very little force. Unless experts unanimously favour B, it shouldn’t be hard to line up 10 who favor A (or even to induce some who are neutral to endorse A). And similarly, it’s nearly always possible to find some criteria on which one option is better.

Exactly the same issues arise in relation to the dangers (or safety) of smoking. The evidence here is statistical, so if you’re looking for logical certainty you won’t find it. And it’s always possible to find some benefits from smoking and some qualified people willing to give a low estimate of risks. But if you rely on the general judgement of independent experts, you’ll reach the conclusion that smoking (direct or passive) is likely to shorten your life and damage your health.

The counterargument, from Whelan and others is that “All scientists have personal ideologies”, and therefore that scientific work should be evaluated on its merits, without regard to source. This sounds appealing until we ask the question “evaluated by whom?” The only people who can usefully do the evaluation are qualified scientists and the only way we can rely on their evaluation is if we believe them to be free of conflicts of interests.

If you accept Whelan’s argument, you end up in a position of complete agnosticism about anything you can’t know from direct experience. She denies this, saying that “If the Tobacco Institute had been funded by the Easter Bunny, its pronouncements would still have been scientifically outrageous, because the controversy had long since ended over whether cigarettes are the primary cause of premature, preventable death ” but, by definition, controversy only ends when one side gives up. (The exposure of the fact that most of the defenders of the safety of smoking were recipients of tobacco money was one of the things that helped induce them to give up.)

As far as the relevant scientific communities are concerned the controversy over evolution has ended and the controversy about climate change has resolved most of the key issues (for example, that warming is taking place and that human activity is a contributor), as has the controversy about the safety of consuming GM foods, but that doesn’t stop people claiming otherwise. And the tobacco lobby only retreated from the glaringly false claim that smoking is harmless to the claim (absurd if you accept that direct smoking causes cancer, but harder to disprove) that passive smoking is harmless. Unless you want to become an expert in biology, geology, climate science, clinical medicine and statisics, among other disciplines, you’ll never be able to resolve these disputes without relying, at some point, on expert judgement.

Obviously, there’s an element of circularity here. We not only have to trust scientists to give us the best advice, but we also have to trust them to tell us who the relevant scientists are. The big argument for accepting this is the undeniable success of the scientific enterprise as a whole, and its demonstrated capacity for correcting error. This can be contrasted with the demonstrated capacity of interest groups to maintain propositions that suit their interests in the face of strong, indeed overwhelming, evidence to the contrary.

fn1. For the fascinating history of Spiked see this Jason Soon post. For Whelan’s own background see Sourcewatch.

108 thoughts on “Ad hominem ad nauseam

  1. I’m sorry to but in Terje but you actually haven’t said anything there. Just what is your position? Do you have an objection to smaller class sizes?

  2. Sdfc,

    Do you have an objection to smaller class sizes?

    Point 5 on my list above was pretty clear. However let me elaborate. I think more teachers for a school would in general lead to better educational outcomes. However it is not clear to me that this would always be the best use of the money. In some schools/subjects it may be more important to have better quality teachers or more books or better fascilities or air conditioning or any number of other things. Broadly speaking I think it is and should remain an ongoing debate. I’d probably prefer that the debate was mostly centred around individual schools rather than on a state wide basis (priorities are different in different places) however either way I think the debate needs to be ongoing as factors will change over time.

    I’m sorry to but in Terje but you actually haven’t said anything there. Just what is your position?

    The issue was about whether teachers who participate in debates about class sizes have a conflict of interest. I argued that they do but should be part of the debate anyway. John Quiggin argued initially that they have no conflict of interest. I have been trying to understand his logic on this latter point but I have not found his reasoning to be at all accessible. He indicated that he would elaborate and I am keen for him to do so.

    My position is that teachers have a conflict of interest in the class size debate. I am waiting for John to refute the logic that leads me to that conclusion. On comment 74 he implied that my logic was actually okay but suggested that he would need to address it in a more detailed response due to some apparent complexity in the choice of model.


  3. “1. I claim that a policy of smaller classes will due to market forces (supply and demand) necessitate higher teachers wages.”

    as in, more demand for labour increases the price of labour, with time-dependent elasticities etc.

    You can’t make an omelet without breaking eggs I guess.

  4. Terje, the obvious problem is that there isn’t a market for education services of the kind you’re assuming. The government is, in effect, a monopsony buyer (note that governments fund most private schools, so they don’t change the picture significantly). So the mechanism you assume simply doesn’t exist here.

    Consider the case when the education budget is fixed. An increase in teachers wages reduces the number employed. Conversely, a policy that requires an increase in the number of teachers can only be achieved by reducing wages (or with budgets increasing over time, forgoing wage increases).

    So, teacher unions face a trade-off between raising wages and increasing teacher numbers.

  5. John,

    If you reduce wages it seems obvious to me that you will have an issue with both teacher recruitment and teacher retension. As such reducing wages will not achieve a reduction in class sizes. It seems to be to be a completely false presumption that you are making.

    Lets say that the government announced that it was going to cut class sizes by 10% and reduce teachers wages by 10% to fund the intitative. How would teachers unions respond? They would be outraged I am sure. Lets say that the government ignored the unions and pushed ahead with the reform. Don’t you think it is self evident that the attrition rate for teachers would increase? Don’t you think it self evident that recruitment would become more difficult? Do you imagine that the number of teachers would actually be increased?

    From where I stand you seem to be lost in a hypothetical model detached from reality. When it comes to the labour market for teachers I accept that the government essentially has a monopoly on the demand side of the equation. However the supply side (ie people who want to teach) is not subject to any such monopoly. A teacher can get a job in any number of other fields if push comes to shove or can move interstate or abroad.

    If however the government can in fact cut the pay of teachers by 10%, increase teacher numbers and maintain the existing quality of teachers why on earth haven’t they just done it? In fact if it could be done on those terms who would oppose it?


  6. Terje, now you seem to assume that teachers are earning the market wage, in which case the position of teachers unions is of no significance.

  7. How do you think the position of teachers unions is significant in this analysis? Maybe it is but I can’t see how.

    If we assume that union muscle (the threat of strikes) means that they have got salaries 10% higher than they would be in the absents of a union it does not seem to alter the dynamics at all. A 10% reduction in wages (back to the non-union scenerio) would still mean that recruitment and retentions was marginally more difficult and expanding the number of teachers would be problematic (unless you cut quality).

    You seem to assume that there is no substitution effect and teachers are in effect bonded to the education department (by superior wages). And if so why hasn’t the government just done it?

  8. PrQ,
    Perhaps putting it this way would help. The government wants to increase university education in the state. It builds a new university and announces that one of the ways it is being funded is to cut the wages of existing staff by 10%. Would this encourage you and your colleagues to seek work in other states or overseas?
    On the other hand, the government realises that Professors are not a dime a dozen and wants to have more in Queensland for the benefit of the university students. Can they simply create more positions that pay more than yours, possibly for staff less published than you are, without you expecting a bit more in your next salary discussions?
    Would you be happy with the union that argued in favour of such wage disparities?

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