In my work I occasionally write about neurons. A common description of the relationship between two populations of neurons is to describe one as being "afferent" or "efferent" with respect to another. One essentially means "downstream" and the other means "upstream," but, for the life of me, I have the hardest time remembering which is which.

However, English boasts another pair of often-confused words that seem quite similar: "affect" and "effect." I'd like to port my understanding of "effect" and "affect" to help me understand how to use the neuroscience terms. Is it useful to try to bridge this gap, or is the similarity here a sort of faux ami?

  • Google "afferent origin", "efferent origin". etc. There's a graph for each...
    – jimm101
    Commented Mar 30, 2016 at 11:35

3 Answers 3


Yes and no, the following entries are from etymonline.com (emphasis mine):

  • effect (n.)
    late 14c., "a result," from Old French efet (13c., Modern French effet) "result, execution, completion, ending," from Latin effectus "accomplishment, performance," from past participle stem of efficere "work out, accomplish," from ex- "out" (see ex-) + facere "to do" (see factitious).

    Meaning "impression produced on the beholder" is from 1736. Sense in stage effect, sound effect, etc. first recorded 1881. The verb is from 1580s. Related: Effecting; effection.

  • efferent (adj.)
    1827, from Latin efferentem (nominative efferens), present participle of effere "to carry out or away, bring forth," from ef- (see ex-) + ferre "to bear, carry" (see infer).

  • ex-
    word-forming element, in English meaning mainly "out of, from," but also "upwards, completely, deprive of, without," and "former;" from Latin ex "out of, from within," from PIE *eghs "out" (cf. Gaulish ex-, Old Irish ess-, Old Church Slavonic izu, Russian iz). In some cases also from Greek cognate ex, ek. PIE *eghs had comparative form *eks-tero and superlative *eks-t(e)r-emo-.

So, in the case of effect and efferent, they share the prefix ef-, derived from ex- but they come from different verbs, the former derives from facere while the latter from ferre.

As for affect and afferent etymonline.com only lists affect:

affect (n.)
late 14c., "mental state," from Latin noun use of affectus "furnished, supplied, endowed," figuratively "disposed, constituted, inclined," past participle of afficere "to do; treat, use, manage, handle; act on; have influence on, do something to," a verb of broad meaning, from ad- "to" (see ad-) + facere (past participle factus) "do" (see factitious). Perhaps obsolete except in psychology. Related: Affects.

According to wictionary, afferent derives:

From Latin adferens (“bringing to”), present participle of adferre (“to bring to”), from ad (“to, toward”) + ferre (“to carry, bear”).

So, afferent and efferent have a common origin and so do effect and affect. However, the ef- and af- pairs only share a common prefix. Still, since that prefix is the same for each pair, you can indeed use effect and affect to help you remember.

  • your conclusion is incorrect. According to dictionary.com both afferent and affect are derived from the same prefix af-. And the same goes for efferent and effect
    – user49727
    Commented Sep 5, 2013 at 15:43
  • 2
    @user49727 please read my answer. That is exactly what I say, they share a prefix but derive from different verbs.
    – terdon
    Commented Sep 5, 2013 at 15:45
  • OK I see what you mean now. In the sense that the prefixes are used they have common derivation - I think this is what OP was trying to establish. That they are different words was not in question I figured.
    – user49727
    Commented Sep 5, 2013 at 15:50
  • @user49727 the prefix of a word is hardly indicative of shared etymology. It is, rather, the word following the prefix that defines it. I would not say that example and exhume are related as such. Yes, they share a prefix but apart from that have nothing in common whatsoever. On the other hand, include and exclude are clearly related despite their different prefixes. The OP is probably aware of the shared prefix, hence the question, it is the rest of the word that is interesting, especially since it turns out they are not related.
    – terdon
    Commented Sep 5, 2013 at 15:55
  • isn't that the way words are formed? by combining roots with particular meanings. In this particular case I think the connotations of both pairs of words are very similar and entirely dependent on the prefix. Wouldn't you agree?
    – user49727
    Commented Sep 5, 2013 at 15:58

As both come from latin, I checked the meanings of the latin origins to get an idea. (My own latin studies are now quite long ago, so I was not sure. Unfortunately the source is in german.)

affectus (affect) means "condition" or "state".
effectus (effect) means "consequence" or "outcome".

affere (verb, origin of afferent) means "bring to".
effere (verb, origin of efferent) means "carry out".

You bring something (in)to a state. You carry something out to get the outcome or consequence.

I'd say, it has a relation. The problem is more the gap between the difference of affectus and effectus but the similarity of affect and effect.

  • 1
    I suggest you do link the source even though it's in German - some here speak German (not me) - but, in any case, actual sources should always be acknowledged.
    – TrevorD
    Commented Sep 5, 2013 at 14:46
  • I did. It's nothing that I really quoted, I just checked if I recall the meanings right.
    – skymningen
    Commented Sep 5, 2013 at 14:51

Excellent question. The two pairs are indeed related etymologically as are many other words sharing the same initial two letters.

ef- is considered cognate with ex- meaning out of as in

effect (n.) from ex- "out" (see ex-) + facere "to do" (see factitious).

af- is considered cognate with ad- meaning to or toward as in

affix (v.) from Medieval Latin affixare, "fasten to, attach," from ad- "to" (see ad-) + figere "fasten" (see fix (v.)).

Looking at the word origins of each word dictionary.reference.com also confirms that the af- and ef- pairs share their prefix origins.

Not only that - at least in medical parlance, effector and efferent are very often used interchangeably. Most commonly to describe the endocrine and immune systems, as in the effector/efferent arm of the immune response.

And the easiest way to remember the meaning of afferent is to remind yourself that it is related to ad- meaning toward. Conversely ef- implies an outward direction.

In conclusion, I hope I have shown that these pairs are somewhat more than faux ami.

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