Often when we present a case to someone else for accepting a particular conclusion, they
will say, ‘Ah, but what about . . .?’, offering some piece of information which we have not
mentioned and which they think weakens our case. In relation to our earlier example
concerning the dangers of smoking, imagine someone saying to you, ‘Knowing that smoking
is dangerous cannot be sufficient to stop people from smoking, because there has been
so much publicity about the health risks, and yet people still smoke’. Let us suppose that a
survey of smokers’ beliefs has been carried out. You might then reply to the above statement,
‘Ah, but what about that survey which showed that, unlike non-smokers, smokers
generally believe that smoking is not bad for one’s health?’ The other person must then
consider what impact this has on their conclusion.
Being able to assess the impact of additional evidence is valuable because people frequently
challenge each other’s reasoning by offering some new piece of information. One response
to such challenges would be to question the truth of the new piece of evidence, and this
would involve one of the skills we have already mentioned – that of evaluating the truth of
evidence or reasons. Another response might be to say that even if the new piece of
evidence were true, it would not weaken the conclusion. This involves the other vital skill
which we have discussed – that of assessing the degree of support which a reason gives to a
conclusion.
Of course, the context may not be one in which we are trying to defend a conclusion – nor
should we be thinking in terms of the necessity to defend a conclusion at all costs. That
would be to indulge in uncritical thinking – being determined to believe something even
in the face of evidence to the contrary. So we must be prepared to acknowledge that
sometimes additional evidence will weaken our conclusions. Sometimes new evidence
comes to light not in the context of a discussion, not when someone else is trying to
undermine one’s own reasoning, but simply in relation to a subject upon which we already
hold an opinion, and believe that we hold that opinion for good reasons. Once we see that
the new evidence is relevant to the issue, we must then consider whether it counts for or
against our earlier opinion – that is to say we must consider whether it strengthens our
reasoning and not merely whether it weakens it.
This exercise gives you practice in evaluating the impact of additional evidence on an
argument. For each of the following multiple choice questions, pick the correct response,
explain why it is the correct response, and explain why each of the other responses is
incorrect.
1 A recent study found that school-age children who participate in school-related
sports activities fight less during school and school-related activities than do those
children who do not participate. It was concluded that sports must satisfy an aggressive
impulse which would otherwise be released through fighting.
Which of the following, if true, weakens the conclusion referred to in the above
passage?
(a) School-related sports activities are always supervised by adults.
(b) Supervisors of school-related sports activities discourage participants from being
extremely aggressive.
(c) Children who participate in school-related sports activities tend to be more
aggressive physically than those who do not participate.
(d) Approximately 85 per cent of the fights children get into during school or
school-related activities take place during break times.
(e) Most schools suspend those who fight during school or school-related activities
from the schools’ sports teams.
(Law School Admission Test, 1982)
2 Although the number of undergraduates studying engineering has grown greatly
over the last five years, there may be a shortage of engineering teachers in the near
future because the number of people receiving PhDs in engineering, those most likely
to teach, has not been increasing. This results because the high salaries offered to
engineers without advanced degrees reduce the incentive to pursue post-graduate
studies. Therefore, businesses will have to recognise that their long-term interests
would best be served by reducing salaries for those without advanced degrees.
Which of the following, if true, would most weaken the above argument?
(a) Enrolment in the sciences has grown over the last five years.
(b) Fewer than half of the people who have received PhDs in engineering teach
full-time.
(c) Businesses pay high salaries to engineers with advanced degrees.
(d) The increases in engineering enrolment are due to the high salaries paid by
businesses.
(e) Many university programmes are funded by businesses interested in engineering
research.
(Law School Admission Test, December 1983)
3 Joan: One method of reducing serious crime in the United States is to adopt the
English system of providing free heroin to heroin addicts.
Anna: That’s absurd. It’s just like giving free cars to automobile thieves.
Which of the following, if true, would most strengthen Joan’s argument?
(a) Heroin addicts are more likely to be violent under the influence of drugs than
when they are anticipating using those drugs.
(b) The amount of money needed annually to supply heroin to heroin addicts is less
than the amount lost annually by the victims of drug-related crimes.
(c) It is cheaper to provide addicts with drugs than to jail them after they have
committed crimes.
(d) The amount of serious crime committed by non-addicts is roughly equal in
England and the US.
(e) A substantial amount of serious crime is committed by heroin addicts in order
to support their habits.
4 Since only 4 per cent of all automobiles fail the state’s annual safety inspection
solely because of defective direction indicators, the state’s automobile association
recommends that direction indicators no longer be inspected. Although they are an
important safety feature, too few are defective to make the expense of testing them
worthwhile.
Which of the following, if true, points out the most serious weakness in the
recommendations of the automobile association?
(a) Owners will no longer maintain their automobile direction indicators in
working order if the inspection requirement is dropped.
(b) Owners of automobiles with defective direction indicators may not have
learned to use manual direction signals.
(c) Eliminating the inspection of the direction indicators will make the state’s
inspection procedure less thorough than those of neighbouring states.
(d) Automobiles with defective direction indicators will fail inspection anyway if
they have other safety defects.
(e) Automobiles that have defective direction indicators may have other defects not
covered by the safety inspection system.
(Law School Admission Test, February 1983)
5 A recent study found that if children watched up to one hour of television a day,
their performance in school was unaffected, but if they watched between two and
three hours a day, they were likely to perform considerably less well than their peers
who watched less. The researchers concluded that if parents carefully monitored the
time their children watched television, the children’s school performance would be
maintained at adequate levels.
If true, which of the following statements about the children in the study would
most strengthen the conclusions of the researchers?
(a) Most of the children who performed at below-average levels in school watched
more than two hours of television a day.
(b) Children who watched television mostly at weekends performed better in
school than children who watched television mostly on school nights.
(c) Children who spent more time reading than watching television performed
better in school than those who did not.
(d) The disparities among the children in terms of school performance lessened
when the television viewing habits of the children became more uniform.
(e) The children who reduced the amount of television they watched daily spent
the extra time reading.
6 It is unwise to continue the career training and employment programmes administered
in most prisons today. These programmes do not achieve what they are meant
to achieve because most ex-prisoners choose not to pursue the occupations they
followed during the time they spent in prison.
Which of the following, if true, most weakens the above argument?
(a) Many habits and skills learnt in prison training programmes are valuable in a
great variety of occupations.
(b) Prisons have an obligation to provide prisoners with occupational training they
will later use in employment.
(c) Prison career training programmes tend to make prisoners more productive
during their time in prison.
(d) Training prisoners for future employment is a major goal of most rehabilitation
programmes today.
(e) In most prisons today, prisoners can prepare for their choice of a number of
occupations.
(Law School Admission Test, 1986)
7 Certain physiological changes accompany the psychological stress of telling a lie.
Reliable lie detection is possible, because, with the appropriate instruments, we can
measure the physiological symptoms of lying.
Which of the following, if true, most weakens the above argument?
(a) Lie detectors are expensive machines, and they require careful maintenance.
(b) Some people find lying only moderately stress-inducing.
(c) Lie detection requires highly trained, capable personnel.
(d) Even the appropriate instrument can be misused and abused.
(e) Numerous kinds of psychological stress produce similar physiological
symptoms.
(Law School Admission Test, March 1984)
8 It is unrealistic to expect flu vaccines to give total protection against the flu virus.
Every winter 12000 people in the UK die as a result of catching flu. The elderly and
those with lung conditions are given priority for flu vaccinations because they are the
people most at risk. Flu vaccines protect against the flu strains judged by the WHO
(World Health Organisation) to be those most likely to be in circulation the following
winter. This prediction is often made a full year before the vaccine is used. If a new
strain of flu appears, or if the current one changes a little, people will not be protected
by the vaccine.
Which of the following, if true, most strengthens the above argument?
(a) The flu vaccine cannot protect people against colds.
(b) If children were given the flu vaccine, the flu virus would spread less rapidly.
(c) The flu vaccine works less well in the elderly than in younger people because the
immune response weakens as people age.
(d) Vaccinating the elderly against flu massively reduces the risk of serious complications
and/or death from the virus.
(e) Many of those who die as a result of flu have not been vaccinated against the flu
virus.
9 The recent experiment on the M42 motorway to provide an extra lane for drivers
at peak times by allowing them to use the hard shoulder has been judged a success.
It is right, therefore, to extend the scheme to other motorways, since it will solve
the problem of congestion on motorways relatively easily and cheaply. It is thus
preferable to other possible measures, such as building new motorways or adding an
extra lane to existing motorways.
Which of the following, if true, most weakens the above argument?
(a) In many countries motorways were built without hard shoulders.
(b) For many journeys, using public transport is less convenient than driving
on motorways.
(c) Environmentalists would be strongly opposed to the building of new
motorways.
(d) It is predicted that motorway traffic will increase by 50 per cent over the next
eight years.
(e) When the hard shoulders are used, there will be a 50mph limit on traffic in all
lanes.
10 It is becoming fashionable to use ‘carbon offsetting’ in order to salve your conscience
about the pollution to which you contribute when taking an airline flight. Companies
have been set up that enable you to buy offsets that, for example, help to build biogas
digesters in India, install hydroelectric power in Bulgaria or distribute energy-efficient
light bulbs in Jamaica. There are two reasons why we should disapprove of this
practice. First, the availability of the scheme makes people think there is nothing
wrong with supporting the carbon polluting aviation industry. If people want to
contribute to such schemes, they could do so without taking a flight. Second, the
companies that offer carbon offsetting are not sufficiently regulated for customers to
know whether the money paid over really does go to the energy saving scheme,
or even whether the scheme really will save energy and thereby reduce carbon
emissions.
Which of the following, if true, most strengthens the above argument?
(a) The Government is working on the production of a benchmark of quality for
carbon offsetting companies.
(b) Carbon emissions from the aviation industry account for only a small percentage
of all emissions.
(c) Helping to finance energy saving schemes in other countries can achieve a
reduction in carbon emissions.
(d) Most people could reduce their carbon emissions by turning down the thermostat
on their central heating.
(e) Some of the schemes to which carbon offsetting contributes would go ahead
even without such a contribution.
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