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Why is it that natural things like farts, poop, halitosis etc., from which we are always surrounded smell "bad"; whereas manufactured products, such as perfume or glue smell "good" to most of us?

In my understanding, things smell "bad", because they are not good for us. But how are farts not good? And shouldn't artificial things like perfume or glue, which are relatively new to us, smell "bad"?

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    $\begingroup$ There are loads of "natural" things that smell good: flowers, food, your lover, your offspring, babies in general (at least to some people anyway), wood... $\endgroup$
    – terdon
    Commented Jan 16, 2014 at 14:17
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    $\begingroup$ feels Good to know that m not the only with a 10 year old's sensibility ( on farty stuff ;) $\endgroup$
    – user4058
    Commented Jan 16, 2014 at 14:33

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Why is it that natural things like farts, poop, halitosis etc., from which we are always surrounded smell "bad"; whereas manufactured products, such as perfume or glue smell "good" to most of us?

In my understanding, things smell "bad", because they are not good for us. But how are farts not good? And shouldn't artificial things like perfume or glue, which are relatively new to us, smell "bad"?

Kind of a strange question but here is a quick and rough answer... There are at least three things to think of here.

Firstly, yes often natural things that are good for you have evolved to smell attractive, but not all natural things are good for you. Therefore a fart should not smell good just because it is natural. And it is likely that breathing farts non stop is not good for you.

Secondly, perfumes etc. have been chemically manipulated or designed to smell nice by human manufacturers - the preference for such smells existed before the specific perfume, the manufacturer simply designed a fragrance that was attractive.

And Finally... Not everything exists because of selection. Farts probably smell as a by-product of process of digestion rather than selection for avoiding them. Gas is produced during the digestion processes, the amount and odor (determined by the constituents of the gas) of gas will likely depend the efficiency of digestion and what is being digested. The dominant selection force affecting the odor is then probably related to digestive efficacy and selection against odor would only be (relatively) very weak.

It's better to digest food and fart a bit than it is to not digest but smell nice, digestion is quite important!

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  • $\begingroup$ Flowers smell good to us. Is there any evolutionary advantage to us or to the flower ? $\endgroup$
    – biogirl
    Commented Jan 16, 2014 at 5:13
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    $\begingroup$ Generally speaking flowers smell good to pollinators - there are a limited number of possible scent chemicals and/or common arrays of "good" smell receptor in the world so I would suggest that there is likely to be some crossover - but that is pure guess work $\endgroup$
    – rg255
    Commented Jan 16, 2014 at 7:09
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    $\begingroup$ Some comments from me: Most substances that smell bad contain some kind of sulphur components - and we are really sensitive to them. If I open a bottle with beta-Mercaptoethanol in a lab (and not in the fume hood) it will only take seconds until someone else starts protesting and opening up windows. Some of these chemicals are highly toxic (for example hydrogensulfide), so it makes sense that we are aware of them. Regarding the flowers: There are some flowers, that smell really awful, have you ever been close to a Ginkgo tree that is blossoming? Not very nice I can tell you... $\endgroup$
    – Chris ♦
    Commented Jan 16, 2014 at 9:20

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