When you think of Christmas, what comes to mind? A beautifully decorated tree? Carols round the fire? Mountains of thoughtfully chosen gifts? Or….like me, is it all about the food?

For me, Christmas wouldn’t be Christmas without the traditional spread- turkey, cranberry sauce, mince pies and, of course, a Brussel sprout or ten. I love Brussel sprouts, and eat them throughout the winter, but to some people this will seem really weird. Brussel sprouts are one of the most polarising vegetables around, with many people finding them so disgusting they just can’t understand why others (like me) would ever choose to eat them. But why is this?

To really understand why sprouts trigger such dramatic reactions in some people, we first have to look at what disgust is. I’d like you to imagine something for me- picture in your mind Donald Trump. Now imagine him wearing a man-kini….

If you have a mirror nearby, take a look at your face. What I assume most of you are exhibiting here is the classic disgust facial expression. This is the same expression you would show if I asked you to picture a piece of rotting meat, crawling with maggots, being held up to your face[1]

This expression was classified by Darwin in his book the Expression of the Emotions in Man and Animals. But why would this response, and this facial expression, have evolved in the first place? When we look at the objects that routinely cause disgust responses, we find they tend to be things that could make us ill; gone-off food, other people’s bodily fluids or animals that are disease vectors like flies. So it seems likely disgust evolved as a way to protect us from touching or eating something that could cause us harm[2].

‘Disdain and Disgust’ from Darwin’s ‘The Expression of the Emotions in Man and Animals’

We don’t know for sure, but one theory suggests that this facial expression encourages us to get further away from the object of our disgust (the recoiling instinct), blocks intake of potentially contaminated air by wrinkling our noses and helps us reject anything unpleasant we put in our mouths as we gape and stick out our tongues.

But what does this have to do with Brussel sprouts? Why would fairy cabbages (as my parents used to call them!) be seen as potentially harmful? To explain that, we have to turn to genetics. Our genes can influence how we taste certain chemicals, and one of these is snappily called phenylthiocarbamide (PTC).  Scientists have discovered a single gene that codes for the PTC receptor; if you have one variant of this gene, PTC tastes very bitter, while another means you can’t detect it at all[i]. Less common versions give in-between reactions.

Ginny Smith and sprouts tasting volunteers at Nine Lessons 2017

PTC is very closely related to a compound found in leafy green veg, including sprouts[ii]. So to some people sprouts taste hugely bitter, while others don’t detect this bitterness at all[iii]. Interestingly, the proportion of non-tasters varies dramatically in different populations- from 50% in Australian Aborigines to less than 10% in central Americans[iv].

Bitterness in plants often signals that they are poisonous, so we have evolved to be wary of bitter things, and extreme bitterness can lead to the protective disgust response we discussed earlier. And as kids are more sensitive to bitterness than adults (perhaps to protect them before they have had a chance to learn which plants are safe to eat) this also explains why so many kids have to be bribed to eat even a single sprout at xmas dinner, even if they are told they are fairy cabbages!

This doesn’t mean your tastes can’t change- many people learn to love things they previously hated through repeated exposure or by paring the food with something they do like (sprouts and bacon anyone?) But if you are a sprout hater, at least now you have an excuse for not eating your greens; you can blame it, at least partly, on your genes.

[1] Not that I’m comparing the leader of the free world with a maggot of course- one is a slimy, wriggling creature you wouldn’t want anywhere near you, and the other is just a fly larvae!

[2] Or bomb north korea…

[i] https://www.theguardian.com/science/blog/2011/nov/01/brussel-sprout-gene

[ii] https://www.compoundchem.com/2014/12/04/brusselssprouts/

[iii] https://www.nature.com/news/2006/060918/full/news060918-1.html

[iv][iv] https://www.nidcd.nih.gov/health/statistics/global-variation-sensitivity-bitter-tasting-substances-ptc-or-prop

Ginny Smith is a science presenter and writer. A Natural Sciences and Psychology graduate of Cambridge, Ginny performs science shows all over the world and presents a wide range of science content for the likes of the Cosmic Shambles Network and the Naked Scientists. She is the co-author of three DK Publishing books looking at science, food and the human body. She is @GinnyFBSmith on Twitter.

If you would like to reuse this content please contact us for details

Subscribe to The Cosmic Shambles Network Mailing list here.