From the definition of the word interest in Macmillan's Dictionary
[countable/uncountable] an advantage or benefit to someone or something
be in your (own) interest(s) (=bring an advantage or benefit):
It's in their own interest to cooperate.
have someone's (best) interests at heart (=want to help them):
He claims he has only my best interests at heart.
be in someone's/something's (best) interest(s) (=bring an advantage or benefit):
The president doesn't believe the plan is in the best interest of the company.
However, The FreeDictionary gives two versions for the idiom "... at heart"
have someone's best interest(s) at heart
to make decisions based on someone's best interests.
I know she was only doing what would benefit her, but she said she had
my best interests at heart.
And from Oxford Dictionaries, it seems that interests is generally more common.
in the interests (or interest) of something
2 For the benefit of:
in the interests of security we are keeping the information confidential
One such principle holds that space is to be explored and used ‘for
the benefit and in the interests of all countries.’
We are doing so because it is in our best interests and in the
interests of our citizens.
The Government should act in their interests and not just in the
interests of the elite who can afford to build nursing homes.
It seems it is a question that may bug some native speakers too!
A relevant question on the Web: question for grammar junkies.....
Is it... "..in the child's best interest?" or "...in the child's best
Does the answer change if you change "child" to "children?"
NB: Since the proof was in Macmillan's dictionary already, I should consult more than one dictionary from now on. In fact, many times I find clearer answers and definitely some examples when they are not provided in Cambridge's. I like Cambridge's look and feel more and it is a decent work to be honest.
This one is even more interesting!
in the interest v. in the interests
With this, the post should come to an end
Best interest or interests?
MIND OUR ENGLISH
The Star Online
, Thursday April 10, 2008
IS IT grammatically correct to say “I have your best interest at
heart” or “I have your best interests at heart”?
Assuming both of the above are correct, in the following sentence,
which would you say works better?
“Your priorities are important so believe us when we say that we have
your best interest(s) at heart.”
1) Both are grammatically correct, but when “interest” means
“advantage” or “benefit”, the plural “interests” is more often used.
2) I would go for “best interests”.