After the therapy, eight children (43%) became able to crawl/move on their back.

Or should I use "on their backs"?

Singular because each child only has one back, or plural because we're dealing with eight backs?

  • 4
    Related: “Only those who qualify will be awarded a certificate” or “Only those who qualify will be awarded certificates”? and the many linked questions on the right hand side of this page.
    – RegDwigнt
    Mar 6 '11 at 16:56
  • I'm more puzzled by the sample size of 18.6 children.
    – Phil Sweet
    Jul 21 '20 at 18:58
  • A key aspect of this one is that elements of an experiment are supposed to be independent. Granularity is inherent in experimental design. Thus there were eight separate events of a child crawling on their back. Contrast this with a business closing causing 25 people to lose their jobs. I don't think it matters much whether the agreement issue crosses a prepositional phrase or clause boundary. The inclusion of the percentage value may well confuse the matter of event discreteness.
    – Phil Sweet
    Jul 21 '20 at 20:22
  • These constructions are often variable, the tendency to use the singular becoming more marked with more fixed phrases. All the books you have been issued have a receipt in the back, which you must sign and hand in. Nov 14 '20 at 15:45

Plural, because we are, in fact, dealing with eight little backs. And a back is a back, no matter how small. A child has a back, but children have backs.

I'm sure others will back me on this.

  • 1
    Not relevant - if several children owned a single turtle between them, it would be "their turtle", not "their turtles". Mar 6 '11 at 17:49
  • @user653: But the OP is not talking about a shared turtle.
    – Robusto
    Mar 6 '11 at 18:01
  • 4
    @Robusto, I think the point @user653 was making is that the fact that "their" is plural does not automatically imply that the word following it has to be plural too.
    – Hellion
    Mar 6 '11 at 18:24
  • @Hellion: Ah, I see. @user653 must be responding to @Fountain, not to my post itself.
    – Robusto
    Mar 6 '11 at 18:31
  • 2
    @Robusto: also, you forgot to mention that while children have backs, baby got back. :-)
    – Hellion
    Mar 6 '11 at 18:46

The semantic, grammatical, and logical arguments clearly suggest it's 'correct' to use the plural, and that's what most people do.

However, despite the fact that I doubt if any style guide endorses the singular, it seems that about 10% of usages for back persist in using the singular.

For reasons which escape me, that 'incorrect' minority rises to nearly 25% when the body part in question is chest. In both cases the evidence strongly suggests the incorrect usage is becoming more widespread.

Personally I believe it's a situation where grammarians backed the wrong horse, and their blind prescriptivism will eventually be defeated. People quite naturally want to use the singular when the number of [body parts, whatever] is immaterial, and only the plurality of [babies, people] is relevant. Increasingly, it seems, they're prepared to do this even at the risk of being considered illiterate. .

  • Great explanation fumble. So, you are saying that it is more natural to use the singular in this case in spite of the grammatical rule, right? Aug 19 '19 at 23:33
  • 2
    In that graph, we have no idea of the surrounding context in which the words "on their chest" and "on their chests" are in. This question is about when the subject is plural. So it is possible that people merely discuss individuals on their chest often which tilts the data when discussing plural subjects.
    – Noah
    Jul 14 '20 at 16:55

It is decidedly the Singular Back. This is because it is the therapy that is being evaluated but the results are singular. The children are not grouped or lined up as in a race to see who can move. The plural children is only used to describe the percentage of those who are able to move. Each child moves only on his or her own back, no other, Singular.

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