How to Identify and Solve Necessary Assumption Questions on the LSAT

You can spot these questions by looking for keywords like "depends," "relies," or "requires" in the question stem. The focus is always on what must be true for the argument to work, not what makes it weak (a key difference from sufficient assumption questions).

If a question asks you to identify a statement that the argument depends on, you’re likely dealing with a Necessary Assumption question. The key is that they focus on what must be true for the argument to hold.

To solve Necessary Assumption questions effectively, start by carefully reading the stimulus. All information is important here, so make sure nothing is glossed over. 

Find the conclusion, or the main argument the author is trying to make, and determine the premises offered in support. 

Then, look for logical gaps between the premises and conclusion. 

Ask yourself: What is missing from this argument? What must be true for these premises to lead logically to this conclusion? 

Once you’ve identified potential gaps, apply the negation test. This involves negating each answer choice and assessing whether it destroys the argument. If negating a statement invalidates the conclusion, that statement is likely a necessary assumption. 

Finally, use the process of elimination to remove answer choices that are irrelevant, overly strong, or unnecessary for supporting the argument.

9 Sample LSAT "Necessary Assumption" Questions With Answer Explanations

Here are nine sample Necessary Assumption questions with a detailed explanation for each answer.

Sample Question #1

“Naturalist: The recent claims that the Tasmanian tiger is not extinct are false. The Tasmanian tiger’s natural habitat was taken over by sheep farming decades ago, resulting in the animal’s systematic elimination from the area. Since then naturalists working in the region have discovered no hard evidence of its survival, such as carcasses or tracks. In spite of alleged sightings of the animal, the Tasmanian tiger no longer exists.”

Which one of the following is an assumption on which the naturalist’s argument depends?

(A) Sheep farming drove the last Tasmanian tigers to starvation by chasing them from their natural habitat.

(B) Some scavengers in Tasmania are capable of destroying tiger carcasses without a trace.

(C) Every naturalist working in the Tasmanian tiger’s natural habitat has looked systematically for evidence of the tiger’s survival.

(D) The Tasmanian tiger did not move and adapt to a different region in response to the loss of

habitat.

(E) Those who have reported sightings of the Tasmanian tiger are not experienced naturalists.

Answer:

A. It’s irrelevant who killed the last Tasmanian tigers. What matters is that sheep farming significantly impacted their population. Even if hunters took out the final two, the core argument remains unaffected.

B. This weakens the argument if true. If carcasses were present but quickly consumed by scavengers, it suggests tigers may have still existed recently. The presence of carcasses would imply recent tiger activity.

C. Negating this statement has little effect. The behavior of one naturalist, especially one unable to conduct fieldwork, doesn't meaningfully impact the broader argument.

D. Correct. If this claim is false, if tigers relocated and adapted elsewhere, the argument collapses. That would indicate sheep farming wasn't solely responsible for their disappearance, weakening the conclusion.

E. This would support the argument but isn't essential. Like C, its negation doesn’t matter much. A single, unreliable sighting, especially under impaired conditions, has little evidentiary value.

Sample Question #2

“Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species. Since mercury that accumulates in a seabird’s feathers as the feathers grow is derived from fish eaten by the bird, these results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago.”

The argument depends on assuming that

(A) the proportion of a seabird’s diet consisting of fish was not as high, on average, in the 1880s as it is today

(B) the amount of mercury in a saltwater fish depends on the amount of pollution in the ocean habitat of the fish

(C) mercury derived from fish is essential for the normal growth of a seabird’s feathers

(D) the stuffed seabirds whose feathers were tested for mercury were not fully grown

(E) the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds’ feathers

Answer:

A. If the birds consumed foods other than fish, that could explain their lower mercury levels, providing an alternative cause. This weakens the argument. Negation: Seabirds consumed at least as much fish in the 1880s as they do now. This is actually a necessary assumption. The original statement should be correct, not its negation.

B. The source of mercury in fish doesn’t matter here, only that mercury is present. This has no bearing on the conclusion.

C. This explains why mercury is found in the feathers, but it doesn’t clarify the change in mercury levels over time, making it irrelevant to the main point.

D. This introduces a possible difference between seabirds today and those from the 1880s, which could weaken the comparison and thus the argument.

E. Correct. If this assumption is false, if preservation methods reduced mercury levels, then the lower mercury could be due to that process rather than environmental differences. That would directly undermine the argument. Negation: The preservation process significantly reduced the mercury content in the feathers.

Sample Question #3

“When exercising the muscles in one’s back, it is important, in order to maintain a healthy back, to exercise the muscles on opposite sides of the spine equally. After all, balanced muscle development is needed to maintain a healthy back, since the muscles on opposite sides of the spine must pull equally in opposing directions to keep the back in proper alignment and protect the spine.”

Which one of the following is an assumption required by the argument?

(A) Muscles on opposite sides of the spine that are equally well developed will be enough to keep the back in proper alignment.

(B) Exercising the muscles on opposite sides of the spine unequally tends to lead to unbalanced muscle development.

(C) Provided that one exercises the muscles on opposite sides of the spine equally, one will have a generally healthy back.

(D) If the muscles on opposite sides of the spine are exercised unequally, one’s back will be irreparably damaged.

(E) One should exercise daily to ensure that the muscles on opposite sides of the spine keep the back in proper alignment.

Answer:

A. The author never claimed that equal exercise alone is enough to align the spine—only that it’s important. Other factors may also be involved, so this doesn’t weaken the argument.

B. Correct. If this isn’t true, the conclusion loses its foundation. Negation: Unequal exercise of spinal muscles does not lead to muscular imbalance. This directly undermines the core reasoning.

C. Similar to A, the author emphasized equal exercise as important, not as the sole requirement for spinal alignment. This doesn't conflict with the argument.

D. This option takes the claim too far. It implies that a single mistake can cause permanent harm, which is unrealistic and unnecessary for the conclusion. Negation: Unequal exercise is generally harmful, but doing it once or twice isn’t permanently damaging.

E. The author never stated how often exercise must occur. Regular exercise could still be effective even if not done daily. Negation: Exercising every other day can still promote a healthy back.

Sample Question #4

“Editorial: The city has chosen a contractor to upgrade the heating systems in public buildings. Only 40 percent of the technicians employed by this contractor are certified by the Heating Technicians Association. So the city selected a contractor 60 percent of whose technicians are unqualified, which is an outrage.”

Which one of the following is an assumption required by the argument in the editorial?

(A) Certified technicians receive higher pay than uncertified technicians.

(B) There are no contractors with fewer than 40 percent of their technicians certified.

(C) Technicians who lack certification are not qualified technicians.

(D) Qualified technicians installed the heating systems to be upgraded.

(E) The contractor hired by the city has personal ties to city officials.

Answer:

A. Pay is irrelevant here, the argument never mentions it. Technician compensation has no bearing on whether they’re qualified. Negation: Certified technicians are not paid more than uncertified ones.

B. Comparing to contractors with fewer certified technicians doesn’t strengthen the claim. What matters is whether more certified technicians means greater qualification. Negation: One contractor in Ecuador has 39% certification, but all others have 40%.

C. Correct. This points out the core flaw: the argument assumes certification equals qualification without proving it. Negation: Technicians may still be qualified even if they lack certification. If this is true, the argument falls apart.

D. Who installed the systems in the past is irrelevant. The focus is on whether current technicians are qualified. Negation: The heating system was originally installed by trained monkeys. This changes nothing about the present argument.

E. A potential conflict of interest doesn’t affect whether the technicians are actually qualified. Negation: The contractor has no connections to city officials.

Sample Question #5

“Biologist: Marine animals known as box jellyfish have eyes with well-formed lenses capable of producing sharp images that reveal fine detail. But the box jellyfish’s retinas are too far forward to receive a clear image, so these jellyfish can receive only a blurry image that reveals prominent features of objects but not fine detail. This example shows that eyes are adapted only to an animal’s needs rather than to some abstract sense of how a good eye would be designed.”

The argument requires assuming which one of the following?

(A) Box jellyfish are the only kind of jellyfish with retinas that do not focus clearly.

(B) Box jellyfish have a need to detect prominent features of objects but not fine details.

(C) Box jellyfish would benefit from having retinas that allowed their eyes to focus more sharply.

(D) Box jellyfish developed from jellyfish whose retinas received clear images.

(E) Box jellyfish use vision as their main means of detecting prey.

Answer:

A. What other jellyfish can or can't do isn’t relevant. The argument is specifically about box jellyfish. Negation: Another species of jellyfish cannot focus clearly. This doesn’t affect the reasoning.

B. Correct. The argument assumes box jellyfish don't need sharper vision, but never actually states what they do need. This assumption fills that gap. Negation: Box jellyfish do need to detect fine visual details. If true, this contradicts the conclusion and undermines the argument.

C. This assumption would actually weaken the argument rather than support it. It suggests better vision would help the jellyfish, which challenges the idea that their blurry vision is sufficient. Negation: Box jellyfish would not benefit from better vision. That makes the argument stronge, so it can’t be the correct assumption.

D. This creates confusion. If ancestral jellyfish had better eyes, it raises the question of why box jellyfish now lack that function, possibly suggesting a loss rather than a lack of need. That distinction complicates the reasoning but doesn’t clearly support or undermine the argument.

E. Alternative detection methods are irrelevant to the claim about vision. The point is whether their current vision meets their needs, not whether they rely on other senses.

Sample Question #6

“The advent of chemical fertilizers led the farmers in a certain region to abandon the practice of periodically growing a “green-manure” crop, such as alfalfa, in a field to rejuvenate its soil. As a result, the soil structure in a typical farm field in the region is poor. So to significantly improve the soil structure, farmers will need to abandon the use of chemical fertilizers.”

The argument relies on the assumption that

(A) most, if not all, farmers in the region who abandon the use of chemical fertilizers will

periodically grow alfalfa

(B) applying chemical fertilizers to green-manure crops, such as alfalfa, has no positive effect on

their growth

(C) the most important factor influencing the soil quality of a farm field is soil structure

(D) chemical fertilizers themselves have a destructive effect on the soil structure of farm fields

(E) many, if not all, farmers in the region will not grow green-manure crops unless they abandon

the use of chemical fertilizers

Answer:

A. Even aside from the reasoning, negating a “most” statement rarely has a significant effect, changing from 51% to 50% is usually trivial. As for the logic: the argument assumes that stopping fertilizer use is necessary for solving the problem, not that it's sufficient. Negation: Only half of farmers who stop using chemical fertilizer choose to grow alfalfa. This weakens the conclusion slightly but doesn’t destroy it.

B. The argument claims alfalfa planting stopped entirely due to fertilizer use. It doesn’t say fertilizers were applied to alfalfa, so this misses the point.

C. Whether something is the most important factor is rarely relevant. Being a major factor is usually enough. The argument doesn’t hinge on relative importance. Example: LSAT scores matter for law school admission, even if they aren’t the top factor.

D. This strengthens the reasoning by reinforcing the connection between fertilizer use and alfalfa abandonment, but it’s not necessary. The core assumption is that fertilizer use prevents alfalfa growth.

E. Correct. If farmers might grow alfalfa even while using chemical fertilizers, then fertilizer use isn’t what’s preventing alfalfa planting, undermining the argument. Negation: Many farmers will grow green-manure crops even if they continue using chemical fertilizers.

Sample Question #7

“In early 2003, scientists detected methane in the atmosphere of Mars. Methane is a fragile compound that falls apart when hit by the ultraviolet radiation in sunlight. So any methane in the Martian atmosphere must have been released into the atmosphere relatively recently.”

The argument relies on the assumption that

(A) Mars had no methane in its atmosphere prior to 2003

(B) all methane in the Martian atmosphere is eventually exposed to sunlight

(C) methane cannot be detected until it has started to fall apart

(D) the methane that the scientists detected had been exposed to ultraviolet radiation

(E) methane in Earth’s atmosphere does not fall apart as a result of exposure to ultraviolet radiation

Answer:

A. Incorrect. The argument focuses on the current presence of methane in Mars’s upper atmosphere, so past conditions are irrelevant.

B. Correct. If this assumption is false (i.e., if some methane in the upper atmosphere is never exposed to sunlight), then the conclusion that methane can’t last long there wouldn’t follow. Negation: Some methane in Mars’s upper atmosphere is never exposed to sunlight.

C. Incorrect. This choice strings together familiar terms from the passage—“methane,” “detected,” and “falls apart”—but doesn’t present a meaningful connection. The argument never states that detection causes methane to break down.

D. Incorrect. The argument assumes that any detected methane must not have been exposed to sunlight yet, or it would have already decomposed. The point is that methane can exist temporarily on Mars, but only until it encounters UV radiation.

E. Incorrect. Earth’s conditions are irrelevant to the argument, which is solely concerned with the behavior of methane in Mars’s atmosphere. Even if methane on Earth is shielded from UV radiation, it doesn’t affect the reasoning about Mars.

Sample Question #8

“Young people believe efforts to reduce pollution, poverty, and war are doomed to failure. This pessimism is probably harmful to humanity’s future, because people lose motivation to work for goals they think are unrealizable. We must do what we can to prevent this loss of motivation and therefore must enable our children to believe that better futures are possible.”

Which one of the following is an assumption on which the argument depends?

(A) Motivating people to work to solve humanity’s problems will enable them to believe that the future can be better and will cause them to be less pessimistic.

(B) Enabling people to believe that better futures are possible will help prevent the loss of motivation that results from pessimistic beliefs about the future.

(C) Optimism about the future is better than pessimism, even if that optimism is based on an illusory vision of what is likely to occur.

(D) If future generations believe that the future can be better, then pollution, poverty, and war will be eliminated.

(E) The current prevalence of such problems as pollution and poverty stems from previous generations’ inability to believe that futures can be better.

Answer:

A. Incorrect. This reverses the logic of the argument. The author claims that instilling hope for a better future will boost motivation, not that motivation proves people believe in a better future. The reverse isn’t required.

B. Correct. This is a key assumption. If it’s false (i.e., if believing in a better future doesn’t improve motivation), the argument collapses. Negation: Allowing people to believe in a better future does not enhance their motivation.

C. Incorrect. This introduces a value judgment, but the argument deals with factual outcomes, whether motivation will be lost, not whether efforts are morally right or wrong.

D. Incorrect. Success isn’t the point here. The author’s argument is focused on encouraging effort from young people, not guaranteeing results. Their success in solving problems is secondary to their willingness to try.

E. Incorrect. The source of current problems is irrelevant to the author’s point. The argument centers on how to motivate young people in the present, not on analyzing the past.

Sample Question #9

“Farmer: Crops genetically engineered to produce toxins that enable them to resist insect pests do not need to be sprayed with insecticides. Since excessive spraying of insecticides has harmed wildlife populations near croplands, using such genetically engineered crops more widely is likely to help wildlife populations to recover.”

Which one of the following is an assumption the farmer’s argument requires?

(A) Use of the crops that have been genetically engineered to resist insect pests in place of crops that have been sprayed with insecticides will cause less harm to wildlife populations.

(B) Wildlife populations that have been harmed by the excessive spraying of insecticides on croplands are likely to recover if the amount of insecticides sprayed on those croplands is reduced even slightly.

(C) Crops that have been genetically engineered to resist insect pests are never sprayed with insecticides that harm wildlife populations.

(D) Use of crops that have been genetically engineered to resist insect pests is no more costly to farmers than the use of insecticides on crops that are not genetically engineered.

(E) If a wider use of certain crops that have been genetically engineered to resist insect pests is likely to help at least some wildlife populations to recover, it is likely to have that effect only because its use will prevent excessive and ineffective spraying of insecticides on croplands.

Answer:

A. Correct. This is a necessary assumption. If it's false, meaning GMO crops cause just as much harm as pesticides, then the argument that they’re a better alternative falls apart. Negation: GMO crops cause at least as much harm as pesticides.

B. Incorrect. This is too weak to matter. Saying GMO crops reduce pesticide use “even slightly” doesn’t support the argument meaningfully; a negligible reduction isn’t enough to make them a better option.

C. Incorrect. The argument hinges on whether GMO crops reduce pesticide use, not whether they eliminate it entirely. A crop can still be sprayed and still use significantly less pesticide overall.

D. Incorrect. The cost of GMO crops is irrelevant to the argument, which is focused solely on their potential to reduce harm compared to pesticides, not on whether farmers can afford to adopt them.

E. Incorrect. This adds an unnecessary requirement. The argument aims to show that GMO crops can help. Requiring that they only help if a separate condition is also met weakens the argument by adding an extra hurdle.

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