The thought that voices betray emotions by means of their tone is as old as human history, but new groundbreaking research now pinpoints the two-way correlation between vowels and feelings.

While it is commonly believed that vocal tones and volume levels can reveal emotions, a German research team demonstrated that this is true, but also in reverse.

Published in the journal Emotion, their paper explores the relationship between the meaning of a word and how it sounds.

It's all in the vowels, according to lead authors Erfurt-based psychologist Dr. Ralf Rummer and the Cologne-based phoneticist Dr. Martine Grice, who say it's possible to change emotions through vowel articulation.

They concentrated on two specific vowel sounds: the sound of the long 'e' vowel (/i:/) -- for example, the double 'ee' sound in "trees" -- and that of the long, closed 'o' vowel (/o:/), as seen in the word "moan," having previously associated them with positive and negative states of mind, respectively.

In the first phase of experimentation, Drs. Rummer and Grice subjected 78 participants to video clips whose content was intended to put them in either a good or a bad mood.

Next, they asked participants to invent and pronounce 10 nonsense words.

Participants who had watched video clips intended to put them in a good mood used the long ‘e' most frequently, just as those subjected to negative clips clung to the long, closed ‘o' vowel (/o:/).

In the 1980s, psychologist Fritz Strack conducted an experiment in which his researchers asked participants to hold a pen in their mouth, the angle of which blocked muscles other than those used for laughing and smiling or their unhappy counterparts.

They then showed participants cartoons.

Those holding the pen between their teeth, which encourages the happy facial expressions, found the cartoons significantly more enjoyable than those holding them between their lips, which engages facial expressions linked to negative emotions.

Inspired by Dr. Strack's experiment, Rummer and Grice embarked on the next phase of their research, calling upon 148 participants, in which they attempted to link the emotional qualities of the aforementioned vowels with facial expression.

Rather than subjecting participants to the somewhat demeaning "pen-in-mouth" test, they asked participants to articulate the long ‘e' vowel, which requires contraction of the zygomaticus "happy" facial muscle, every second during a cartoon.

Another group of test subjects was asked to do the same, while pronouncing the long, closed ‘o' vowel, which requires use of the "sad" facial expression muscles.

The same cartoons were enjoyed to the utmost by those who were asked to pronounce the long ‘e' vowel.

In conclusion, Rummer and Grice's work offers evidence that could explain the phenomenon that positive words such as ‘pleased' tend to contain the long ‘e' and negative words such as ‘alone' are likely to contain the long, closed ‘o'.

Both researchers say they hope linguists and language users will take this paper into account for future research.

"We are currently looking at the effect of perceived vowel type on emotional state. Subjects will be rating cartoons, like in the production experiment," said Grice to Relaxnews via email, when asked about future research projects. "If they hear /i/, will they still feel in a better mood than if they hear /o/? Will this mood affect the ratings in cartoons to the same extent?"