Learn English – Where did the expression “it’s lonely at the top” come from

The only reference book I've found that discusses "it's lonely at the top" is Nigel Rees, A Word in Your Shell-like (2004):

(it's) lonely at the top An observation that might apply to any leader prominent politician or show business star. Date of origin uncertain. 'Oh, it's lonely at the top' is the refrain of a song 'Lonely at the Top', by Randy Newman (1972). Apparently, the German philosopher Arthur Schopenhauer (1788–1860) once wrote, 'Auf der Höhe muss es einsam sein [at the height it must be lonely].'

A Google Books search turns up very few instances of the phrase in its modern idiomatic sense before the 1950s, though one instance of "lonely at the top" in what appears to be the modern sense dates back to 1924. From Anne Bryan McCall "A Familiar Tower Room Dilemma Discussed," in Woman's Home Companion (1924) [combined snippets]:

Dear Anne Bryan McCall: I am an out-and-out business girl. I love my work. I eat and sleep and drink it; partly I suppose because I am so successful. Please don't think I am boasting; I just want you to know the facts. I have been in business five years, and am twenty-three. I get a very good salary, and a small share in the profits. In eight or ten years I ought to be making from eight to ten thousand at least. My employers say I can go right to the top.

But that's it! Won't it be lonely "at the top," maybe, when I'm say about forty?

A somewhat less exact match appears in Proceedings of the American Transit Claims Association (1927), in a context where being "at the top" is undesirable whether lonely or not:

Let us talk it over and maybe through our united efforts some of you can point a way for us who are lonely at the top of the list of damage claim percentages to get down among the 3 per cent fellows, where there is more company.

From "Backstage in Washington" in Outlook and Independent (1930) [combined snippets]:

Though it is not generally known, or even suspected, Vice-President Curtis has discovered that it is always lonely at the top. His old poker and racetrack pals have fallen away since he became Vice-President and began to press his pants as well as polish his shoes. The tailor and the bootblack, we suspect, have robbed him of his human appeal. Hiram Johnson we must rank on our lonesome list, but his native instinct for loneliness deprives him of our sympathy; like Norris, he would be positively unhappy if he had any close friends.

From an unidentified story by Rosalind Constable in New English Review Magazine (1949) [combined snippets]:

"Has it ever occurred to you I might be lonely?"

"No", said Marian, "but now you mention it I can see how it could be".

"It can be very lonely at the top of the tree", said Mr. Sherman. And there was a suspicion of a break in his voice.

The first Google Books instance of the exact phrase "it's lonely at the top" in its modern idiomatic sense may be from a 1952 issue of Motion Picture magazine, which supposedly contains this entry in its table of contents:

It's Lonely At the Top Jane Wyman has everything, except what she wants most . . [by] Pauline Swanson [page] 56

Unfortunately, the linked snippet view doesn't show the relevant excerpt from the magazine, nor is the publication date firm.

The first confirmed Google Books match that I could find for "it's lonely at the top" is in the entry for the actor Aldo Ray in Cleveland Amory, Celebrity Registry: An Irreverent Compendium of American Quotable Notables (1959):

People have always sharply divided on his talent. He has been befriended by one of New York's most prominent columnists; on the other hand he himself has said: "I lead a lonely life ... It's very lonely at the top, but the view is good ... Friends are so vital to me ... I must have friends ... and when I had some trouble— you know?— I found out who my friends were. I had maybe this many" — he showed 10 fingers— "and none in show business. No performer, I mean ..."

It seems probable that idiomatic usage of "it's [or it is] lonely at the top" has been around since the 1920s at least. But as this Ngram chart for the years 1900–2005 suggests, the phrase's popularity really began to take off right around the time that Randy Newman's mock ode to the self-pity of the successful, "Lonely at the Top," appeared: