TL;DR: The “double being” of Jekyll and Hyde is a floating signifier, so a Scottish interpretation of the novella could be made. But no-one has done so yet! Wikipedia and James Campbell are mistaken on this point.
Stevenson wrote that the theme of Strange Case of Dr Jekyll and Mr Hyde is
that strong sense of man’s double being which must at times come in upon and overwhelm the mind of every thinking creature
Robert Louis Stevenson (1888). ‘A Chapter on Dreams’. Scribner’s Magazine, January 1888. Collected in Across the Plains, 1892.
Henry Jekyll describes, in his “full statement of the case”, that he was always disposed to “pleasures” that required “profound duplicity” to conceal:
And indeed the worst of my faults was a certain impatient gaiety of disposition, such as has made the happiness of many, but such as I found it hard to reconcile with my imperious desire to carry my head high, and wear a more than commonly grave countenance before the public. Hence it came about that I concealed my pleasures; and that when I reached years of reflection, and began to look round me and take stock of my progress and position in the world, I stood already committed to a profound duplicity of life.
Robert Louis Stevenson (1886), Strange case of Dr Jekyll and Mr Hyde, p. 106, London: Longmans.
Through his scientific experiments, Jekyll is able to make this “duplicity” from a metaphor into a fact, splitting his self into two so that one part can indulge and the other repress. But the novella never explains what Jekyll’s “pleasures” were, and so the meaning of this split cannot be nailed down: it is a reference without an object, that is, a floating signifier.
How can we interpret the dualism of Jekyll and Hyde? Do they represent good and evil? civilization and savagery? public and private? sobriety and drunkenness? chastity and licentiousness? health and disease? law and criminality? Scotland and England? It’s in the nature of a floating signifier that the reader has to bring some idea from outside the text in order to make sense of it.
A gallimaufry of interpretations
Thus, if you bring Stevenson’s own alcoholism and the Victorian alcohol debate, then you can read the text as an allegory of alcoholism:
The cloud of alcohol shadows Strange Case of Dr Jekyll and Mr Hyde as thoroughly as it did its author […] To state the case most simply—and in the novella’s own terms—Stevenson’s best-known tale documents the fate of a man who regularly escapes from an over-burdening respectability into an addictive world of recreational excess by drinking a “blood-red liquor” whose effects “braced and delighted [him] like wine”—but which ultimately reduce him to the same crushing self-recrimination that is suffered when “a drunkard reasons with himself upon his vice” (emphases added). If J&H is not actually an allegory of alcoholism, of the disruptive “Hyde” that strong drink can release in any human being, then it is everything but that.
Thomas L. Reed, Jr. (2006). The Transforming Draught: Jekyll and Hyde, Robert Louis Stevenson and the Victorian Alcohol Debate, pp. 1–2. London: McFarland & Co.
If you bring knowledge of the psychology of addiction, then you can read the text as an allegory of drug addiction:
Denial of the addiction is illustrative of the addict who cannot confront his illness. Such inability frequently is attributable to the addict’s belief that his disability is not physiological but moral. To acknowledge addiction is to acknowledge that one is dependent rather than self-reliant, and addicts “fiercely resist admitting dependency.” To the extent that the addict cannot comprehend his behavior in termsdivorced from the rhetoric of personal and moral failure, the addict likely will refuse to admit that he cannot govern his condition. Jekyll’s reaction to Hyde, the emblem of his addiction, is typical; as he proclaims to Utterson, “to put your good heart at rest, I will tell you one thing: the moment I choose, I can be rid of Mr. Hyde”. The addict untutored in the pathology of addiction will always so mistakenly suppose that he can regulate the use and effects of his intoxicant. Of course, he cannot.
Daniel L. Wright (1994). ‘“The prisonhouse of my disposition”: a study of the psychology of addiction in “Dr. Jekyll and Mr. Hyde”’. Studies in the Novel 26:3, p. 255.
If you bring the pseudo-scientific theories of Cesare Lombroso then you can read the text as an allegory of criminal atavism:
Equating the criminal with atavism, and both with the lower classes, was a familiar gesture by the 1880s, as was the claim that deviance expressed itself most markedly through physical deformity. Stevenson’s middle-class readers would have had as little trouble deciphering the features of the “abnormal and misbegotten” Hyde, his “body an imprint of deformity and decay,” as Stevenson’s middle-class characters do. “God bless me,” exclaims Utterson, “the man seems hardly human. Something troglodytic, shall we say?”
Stephen D. Arata (1995), “The Sedulous Ape: Atavism, Professionalism, and Stevenson’s Jekyll and Hyde”, Criticism 37:2, pp. 233–259.
If you bring Freud’s theories, then you can read the text as an allegory of male sexual repression:
The first incident that is reported to us in Stevenson’s Strange Case of Dr Jekyll and Mr Hyde (1886) as an indication of Hyde’s ‘furious propensity to ill’ concerns ‘a girl of maybe eight or ten’ who collides with him in the street one night: ‘the two ran into one another naturally enough at the corner; and then came the horrible part of the thing; for the man trampled calmly over the child‘s body and left her screaming on the ground’. A number of contemporary readers were dissatisfied with this: ‘the first incident which is meant to show the diabolical character of Mr Hyde is inadequate’, commented the anonymous reviewer in The Athenaeum. […] [Gerald Manley] Hopkins wrote to [Robert] Bridges: ‘the trampling scene is perhaps a convention: he [Stevenson] was thinking of something unsuitable for fiction’. Random violence, in other words, has replaced a sexual drive which it thus serves to express; the nocturnal settings, the not-quite-rightness, the feeling of some inadequacy are so many signs of this (the little girl and the gentleman really should not be where they are in the text). Stevenson would no doubt have protested to Hopkins as he protested elsewhere: ‘people are so filled full of folly and inverted lust, that they can think of nothing but sexuality’; for him, ‘the beast Hyde’ who is let out ‘is no more sensual than another, but … is the essence of cruelty and malice, and selfishness and cowardice: and these are the diabolic in man’. The protest has a kind of self-defeating truth: if people are so filled full of folly and inverted lust, this must be central for what Hyde represents, all those ’lower elements’; and when Jekyll first tries his experiment, he is indeed conscious before all else of ‘a current of disordered sensual images running like a mill race in [his] fancy’.
Stephen Heath (1986). ‘Psychopathia sexualis: Stevenson’s Strange Case’. Critical Quarterly 28, p. 93–94.
If you bring knowledge of Victorian treatment of gay men, then you can read the text as an allegory of male homosexuality:
In a certain way, Jekyll is ashamed of such pleasures and desires, thus partly agreeing with the society that abhors them. He apparently suffers a sense of guilt for his double life. Could we not argue that his duplicit life is a representation of the double life many homosexual men were obliged to conduct in the last decades of the nineteenth century? Could we not also argue that what Jekyll refers to as morbid pleasures are actually the secret encounters many homosexual men indulged in, in the very same foggy streets of London described in Stevenson’s novel?
Antonio Sanna (2013). ‘Silent Homosexuality in Oscar Wilde’s Teleny and The Picture of Dorian Gray and Robert Louis Stevenson’s Dr Jekyll and Mr Hyde’. Law & Literature 24:1, p. 27.
A Scottish interpretation?
Similarly, if you bring some knowledge of Stevenson’s upbringing in Edinburgh, and a concern with Scottish nationalism, then you could read the text as an allegory for the dual nationality and dual language of the Scottish people. But has anyone actually done so? Wikipedia gives no detail of how this reading is supposed to work, saying only:
Another common interpretation sees the novella’s duality as representative of Scotland and the Scottish character
Wikipedia. ‘Strange Case of Dr Jekyll and Mr Hyde’. As of 2019-04-16T19:33:57.
Wikipedia sources this claim to David Daiches and Jared Campbell. But there is no trace of it in Daiches, who writes instead:
It is of course a story about good and evil subsisting within the same person and shows Louis’ lifelong fascination with the ambiguities of the individual moral character. […] The fact that Hyde, the doctor’s stunted evil self, manifests his evil in acts of physical brutality rather than in, say, sexual license, although the moral defectiveness to which he owed his release relates to the kind of hedonistic adventurism that Stevenson himself practised in his bohemian days, is perhaps evidence of Stevenson’s instinctive awareness of the relationship between sensuality and sadism, but it is more likely to result from the impossibility in the 1880s of describing acts of violent sensuality with the frankness they require if they are to administer to the reader the kind of shock which the story demands.
David Daiches (1973). Robert Louis Stevenson and his World, p. 68. London: Thames and Hudson.
James Campbell’s 2008 article in The Guardian (quoted in the question) only gives us an outline of this interpretation, not even indicating whether we are to interpret Jekyll as Scotland and Hyde as England, or the other way round. Campbell refers us to Clayton Hamilton and G. K. Chesterton for the details. But on inspection, neither of those writers makes this interpretation! Hamilton just notes that the setting has a Scottish flavour, and does not attempt to interpret the text at all:
I have often wondered why the Strange Case of Dr. Jekyll and Mr. Hyde was set in London instead of Edinburgh. Utterson is a very Scottish sort of lawyer; Lanyon is a very Scottish sort of doctor; and the metaphysical speculation that allures Dr. Jekyll to his doom is decidedly more Scottish than English. Furthermore, the tale might most appropriately be conceived as happening among the gloomy doorways and mysterious wynds that undermine the tall, decaying lands which darkly overhang the High Street of Edinburgh. Possibly Louis may have felt that Mr. Hyde could lose himself more easily among the shifting crowds of a vaster and less centred city. It is more difficult to hunt a villain down in London than in Edinburgh.
Clayton Hamilton (1915). On the Trail of Stevenson, p. 61. New York: Doubleday, Page & Co.
Chesterton also notes the Scottish aspects of the setting, before moving on to discuss the text as an allegory of good and evil:
There is indeed one peculiarity about that grim grotesque which I have never seen noted anywhere; though I dare say it may have been noted more than once. It will be realised that I am not, alas, so close a student of Stevensoniana as many who seem to think much less of Stevenson. But it seems to me that the story of Jekyll and Hyde, which is presumably presented as happening in London, is all the time very unmistakably happening in Edinburgh. More than one of the characters seem to be pure Scots. Mr. Utterson, the lawyer, is a most unmistakably Scottish lawyer, strictly occupied with Scots Law. No modern English lawyer ever read a book of dry divinity in the evening merely because it was Sunday.
G. K. Chesterton (1927). Robert Louis Stevenson, chapter 3.
Here the chasing of citations reaches a dead end. Far from being “The most popular allegorical reading in our own day”, as Campbell has it, or even a “common interpretation”, as Wikipedia more cautiously puts it, the Scottish interpretation of Jekyll and Hyde remains to be made.