Use my brain or brains?

I think it's mainly due to one ("use your brain") being a simple description, and the other ("rack your brains") being an idiom. But no doubt there has been a bit of cross-fertilisation between the two.

In both American and British books, "use my brain" is more popular than the plural version.

But with "rack" it's a bit different. The idiom often seems to be listed in dictionaries etc as "rack your brains" (although both are allowed):

http://www.phrases.org.uk/meanings/rack-your-brains.html

However, while in British English "rack my brains" is still more popular than the singular version, in American books the singular seems to have overtaken the previously more popular plural version:

http://books.google.com/ngrams/graph?content=rack+my+brain%3Aeng_us_2012%2Crack+my+brains%3Aeng_us_2012%2Crack+my+brain%3Aeng_gb_2012%2Crack+my+brains%3Aeng_gb_2012&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Crack%20my%20brain%3Aeng_us_2012%3B%2Cc0%3B.t1%3B%2Crack%20my%20brains%3Aeng_us_2012%3B%2Cc0%3B.t1%3B%2Crack%20my%20brain%3Aeng_gb_2012%3B%2Cc0%3B.t1%3B%2Crack%20my%20brains%3Aeng_gb_2012%3B%2Cc0

At Google Books I can find no "rack" examples before 1700, but between 1700 and 1750 there are 12 examples each for "brains" and "brain", although in the second half of the 18th century "rack my brain" became more popular. It seems that the "brains" version became more popular in the 20th century - 61- 46 examples at Google Books.

As to your final question I don't think they have any different meaning, it's simply a matter of idiom - "Use your brain" sounds more idiomatic to me, but conversely, so does "rack your brains".

Incidentally, there's another discussion going on about "rack your brains" or "wrack your brains" (Oxford allows both) at DailyWritingTips, StackExchange, and WorldWideWords.

I thought "brains" in the plural tends to mean something like "intellectual capability" whereas "brain" might suggest the physical literal organ. Check out the specifically plural connotations:

http://en.wiktionary.org/wiki/brains

http://en.wiktionary.org/wiki/brain#English

I would think that "use your head" would be more commonly used than either of the brain versions.
But maybe that's a Scottish thing.

:-))

@jayles the unwhateverwillitbenext - as a general idea, I'd agree with you:

"She's the brains of the family"

But Wiktionary gets it wrong when it says plural only for British English to mean intelligence. Oxford and Longman have 'usually plural', Macmillan 'often plural'.

And there are quite a few exceptions:
"She has an amazing brain" (more likely than "amazing brains")

"to blow somebody's brains out"

I don't see much difference in the meaning of brain(s) in the two expressions Dyske is talking about, yet in British English "rack your brains" is much more common in books than the singular version (according to Ngram) , whereas "use your brain" is marginally more common than the plural in BrE, and much more common in AmE .

What is also evident is that there is a movement towards the singular for "intelligence", especially in American English, where "use your brain" is nearly three times as common (in Ngram books) as the plural version, and even "rack your brain" is overtaking the plural version in AmE. In fact over the whole history of this idiom "rack your brains" and "rack your brain" have jockeyed for poll position.

http://books.google.com/ngrams/graph?content=use+your+brain%3Aeng_us_2012%2Cuse+your+brains%3Aeng_us_2012%2Cuse+your+brain%3Aeng_gb_2012%2Cuse+your+brains%3Aeng_gb_2012&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cuse%20your%20brain%3Aeng_us_2012%3B%2Cc0%3B.t1%3B%2Cuse%20your%20brains%3Aeng_us_2012%3B%2Cc0%3B.t1%3B%2Cuse%20your%20brain%3Aeng_gb_2012%3B%2Cc0%3B.t1%3B%2Cuse%20your%20brains%3Aeng_gb_2012%3B%2Cc0

@Hairy Scot - Ngram agrees with you, about two to one, in both BrE and AmE.

An afterthought: couldn't these expressions be referring both to intelligence and the physical organ? I don't think the difference is so clear cut.

Male brains are of course about 10% larger than female brains. Notably, male brains contain about 6.5 times more gray matter than women. Female brains have more than 9.5 times as much white matter, The frontal area of the cortex and the temporal area of the cortex are more precisely organized in women, and are bigger in volume.

Perhaps that explains why "brains" in the plural is increasingly used when referring to women:

http://books.google.com/ngrams/graph?content=her+brain%2Chis+brains&case_insensitive=on&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t4%3B%2Cher%20brain%3B%2Cc0%3B%2Cs0%3B%3Bher%20brain%3B%2Cc0%3B%3BHer%20brain%3B%2Cc0%3B.t4%3B%2Chis%20brains%3B%2Cc0%3B%2Cs0%3B%3Bhis%20brains%3B%2Cc0%3B%3BHis%20brains%3B%2Cc0

Took awhile to find the required statistics!

I would agree that brains implies the substance as a metaphor for intelligence. Looks a count versus uncount issue, such as with the word gut, e.g., It takes a strong gut to eat that cooking, It takes a lot of guts to eat that cooking.

@jayles - "Perhaps that explains why "brains" in the plural is increasingly used when referring to women"

According to Ngram, the increase since 1970 has been less than 100% and the current level is less than it was in 1930, historically having been pretty flat. Meanwhile the instances of "her brain" have increased over the same period by about 400%, and outnumber "her brains" by about 10 to 1, so I'm not quite sure what you're saying here.

http://books.google.com/ngrams/graph?content=her+brain%2Chis+brains%2Cher+brains%2Chis+brain&case_insensitive=on&year_start=1800&year_end=2000&corpus=17&smoothing=3&share=&direct_url=t4%3B%2Cher%20brain%3B%2Cc0%3B%2Cs0%3B%3Bher%20brain%3B%2Cc0%3B%3BHer%20brain%3B%2Cc0%3B.t4%3B%2Chis%20brains%3B%2Cc0%3B%2Cs0%3B%3Bhis%20brains%3B%2Cc0%3B%3BHis%20brains%3B%2Cc0%3B.t4%3B%2Cher%20brains%3B%2Cc0%3B%2Cs0%3B%3Bher%20brains%3B%2Cc0%3B%3BHer%20brains%3B%2Cc0%3B.t4%3B%2Chis%20brain%3B%2Cc0%3B%2Cs0%3B%3Bhis%20brain%3B%2Cc0%3B%3BHis%20brain%3B%2Cc0

It's the same story at the British National Corpus - "her brain" - 268, "her brains" - 23, "his brain" - 230, "his brains" - 44. I don't think you can really draw any conclusions based on gender here.

This is the kind of idiomatic quirk which drives no-English speakers crazy. But, yes, it can definitely be pluralised.

After reading these comments, my brains are more confused than ever before.