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The Harry Potter books have sold more than 450 million copies since the first was published in 1997. 

And now researchers have discovered its success may lie in the use of words author J.K Rowling chose across the series - rather than the texts as a whole.

By scanning the brains of participants as they read passages from the collection, researchers discovered that the common use of 'arousing words' affected parts of the brain concerned with emotion.

The Harry Potter books (pictured) have sold more than 450 million copies and its success may lie in the use of words used - rather than the texts as a whole. By scanning the brains of people reading passages from the collection, researchers discovered 'arousing words' affected parts associated with emotion

The emotional potential of words is rated in terms of valence and arousal ratings. 

Valence refers to how positive or negative a word is, while arousal refers to its 'physiological intensity', or the extent to which it affects the reader and their view of the word or text.

These properties are normally used for words in isolation, which can be categorised as negative, neutral, and positive.

But the researchers from Lancaster University wanted to see how these classifications and findings for individual words could be applied to passages of text and books as a whole. 

'A text is more than a list of words, and the way these words are combined, or the context they are embedded in, clearly matter,' explained the researchers. 

They screened all seven Harry Potter novels: Harry Potter and the Sorcerer's Stone (1997), Harry Potter and the Chamber of Secrets (1998), Harry Potter and the Prisoner of Azkaban (1999), Harry Potter and the Goblet of Fire (2000), Harry Potter and the Order of the Phoenix (2003), Harry Potter and the Half-Blood Prince (2005) and Harry Potter and the Deathly Hallows (2007). 

Study screened seven Harry Potter novels. Emma Watson (left) as Hermione Granger and Daniel Radcliffe are shown in Prisoner of Azkaban

From this, they selected 120 passages, each of which was four lines long and did not require a high level of familiarity with the books. 

In each case, the emotional connotations of the passages were at the start and and the emotional contents were unambiguous and consistent throughout the passage.

The team then used a magnetic resonance imaging scanner to map activity in the brains of people reading these 120 passages. 

Following the experiment in the scanner, each participant rated the passages on valence, on a scale of -3 for very negative to +3 for very positive, and arousal, on a scale of 1 for very calming to 5 for very arousing.

The sentence: 'And then a silence fell over the crowd, from the front first, so that a chill seemed to spread down the corridor', from Harry Potter and the Prisoner of Azkaban featured a range, or span, of arousal.

The researchers said this was due to the contrast between the low arousal of 'silence' and the high arousal of 'chill', which led to the overall lexical arousal of the whole sentence to be moderate.

Meanwhile, the passage: 'Ginny glanced round, grinning, winked at Harry, then quickly faced the front again. Harry's mind wandered a long way from the marquee, back to afternoons spent alone with Ginny in lonely parts of the school grounds,' from the Deathly Hallows was rated positive.

But its valence was neutral. 

The emotional impact is said to result from the drift of Harry's mind into the past remembering his relationship with Ginny and the fact the reader has to imagine it, rather than being told about it.

And the sentence from the Half-Blood Prince: "You disgusting little Squib, you filthy little blood traitor!" roared Gaunt, losing control' contains words that are consistently negative in valence and high in arousal, such as 'disgusting', 'filthy', 'blood', 'traitor', 'roar', and 'losing'.

Following the experiment, each participant rated the passages on valence - from -3 for very negative to +3 for very positive - and arousal - scaled from 1 for very calming to 5 very arousing. Examples are shown 

The team then used a magnetic resonance imaging scanner to map activity in the brains of people reading these passages from the books before asking them to rate each on valence and arousal. A passage involving Ginny (played by Bonnie Right pictured left), and Harry (Daniel Radcliffe right) had a neutral valence

The MRI scans revealed that reading passages like the one from the Half-Blood Prince correlated with brain activity in regions associated with emotion, with building a mental model of a situation and also with the understanding of a character's state of mind.

In particular, these emotional passages stimulated the left amygdala of the brain, which processes emotional reactions, and the insula.

The more emotionally arousing words a text contained, the more it was judged to be emotionally affecting.

But the sentence: 'You disgusting little Squib, you filthy little blood traitor!' from the Half-Blood Prince was said to contain words that are consistently negative in valence and high in arousal. This stimulated the left amygdala of the brain (left) and the insula (right) which are associated with emotional reactions

Thus, the researchers said the emotional engagement readers show is mostly driven by the single emotional words contained in the text, rather than by each passage as a whole.

Psychologist Dr Francesca Citron from the university said: 'These results suggest that a text's constituting words can predict its emotion potential.' 

'When we read a text, specific words reverberate in our minds beyond the more complex message conveyed by the text; the art of choosing the right words with the appropriate affective impact is part of what defines the skill of good writers or speakers.' 

The study: 'The emotion potential of words and passages in reading Harry Potter – An fMRI study' is published in Brain and Language.