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Affect and effect have similar meanings and similar pronunciations, making it sometimes difficult to know which one to use. Generally speaking, people can tell the difference by looking at what grammatical role the word is playing: affect is almost always used as a verb, while effect is usually used as a noun. Some people also use tricks to help them remember, like mnemonics. There are also several exceptions to the common uses of affect and effect, mainly in the context of psychology.
Common Uses of Affect
Affect is primarily used as a verb. It has two main meanings, which are closely related. Most commonly, it means to influence someone or something in either a physical or psychological way, as in "The painting affected him so deeply he could not speak," or "The dam breaking affected the water level downstream." It can also mean to pretend to feel or think something, such as "She affected an interest in art," or "He affected to understand what she was saying, even though he was actually confused."
Common Uses of Effect
The word effect is used primarily as a noun. It has a number of related meanings, but generally speaking, it refers to the result of something, the power something might have to get a specific result, or a natural phenomenon. For example, "The effect of the election was a new balance of power" suggests a direct result, while "The effect of the painting on him was to inspire his own work" focuses on the power needed to make something happen. Phenomena are often also referred to as effects, such as "the photoelectric effect" or "the greenhouse effect."
Exceptions for Affect
Affect is rarely used as a noun, usually in the context of psychology or sociology. In this case, a person's affect is his or her mood or mental state. Having a lack of emotion, called a flat affect, is actually a symptom of several mental conditions, including schizophrenia. Even more rarely, affect is used archaically as a word for an affectation someone might take on — a pose, or disposition, such as "Her affect was one of learning and grace, but those of us who knew her knew better." In both of these cases, the emphasis is on appearance: what type of mood a person appears to be in, and the way a person appears to behave, even though his or her real feelings or personality may be different.
Exceptions for Effect
Effect is occasionally used as a verb as well, usually to refer to something directly causing something else. Grammatically, it takes an object and often has an "-ed" ending added. For example, a person could say "The election at last effected the change the people had been hoping for," using effect as a verb to mean "caused." This use is somewhat more common than the noun uses of affect, but is still much less common than effect's normal use as a noun.
Since affect and effect generally stick to specific grammatical roles, readers can often know which one to choose by identifying the word's purpose in a sentence. Some people also use mnemonic devices to help them remember, like that a verb is an action word, and both action and affect begin with the letter "a". One of the most commonly made errors is substituting effecting or effected for affecting and affected, since the difference between them is subtle: the first means to directly cause something, while the second means to have an influence on something. When the intent is unclear, choosing affect as a verb is much more likely to be correct.