The "Jackie Effect" refers to an unexpected, involuntary emotional response experienced when when listening to a vocal performance.|
The effect has been reported most recently in association with American classical crossover singer Jackie Evancho, but has also been described for other singers:
"I cried ... because it renewed my faith in human potential and opportunity. No one thinks old people can start a new career and become wildly sucessful in it" - Jamie 
"..noticed Jenny's power to draw tears of joy, by her singing. Ever in her voice rang the sympathetic vibrations, at which the tears flow" 
DescriptionThe effect is commonly manifested in goosebumps, spine tingling and tears.
Those impacted typically describe their emotional responses in terms such as:
Examples are reported here and here.
AnalysisStudy of the Jackie Effect is typically undertaken in the domain of Music and Emotion which analyses the impact of tempo, rhythm and tonality on the emotional state and memory of listeners. The distinguishing characteristic of the Jackie Effect is the involuntary or unexpected nature of the response.
Early analysis pointed to the impact of the "Little girl - big voice" paradox as a possible explaination.
However, the effect continues to be reported despite Evancho's maturing persona.
Music and EmotionsMusic and Emotions (Wikipedia) explores emotional impact in three categories:
Performance features comprising performer skills (the ability and appearance of the performer including physical appearance, reputation and technical skills) and performer state (the interpretation, motivation, and stage presence of the performer.)
Reviewer comments often note Jackie's unusual talent for emotional interpretation, which utilises her natural and prodigeous vocal skills to create a hightened emotional state in listeners.
Jackie's off-stage persona of genuine humility and kindness also resonates strongly with her fans.
Listener features referring to the individual and social identity of the listener such as personality, knowledge of music, and motivation to listen.
Audiences currenly comprise mainly middle aged or older men and women in equal numbers, with a smaller number of younger fans. Those trained in music, including orchestra members performing with Jackie, are often glowing in their reviews, but those with less musical knowledge are equally likely to report the effect.
Contextual features which are aspects of the performance such as the location and occasion.
Jackie is a popular performer at patriotic events such as Memorial Day and Fourth of July concerts, which contribute an emotionally heightened atmosphere. These events seem to bring the effect into greater focus, and the resulting successes suggest that emotional connection forms a significant part of the Jackie Effect.
Waveform AnalysisWaveform analysis has identified an unusual double harmonic which may be associated with the effect.
From an article by DLDEPREZ :
"Evancho is able to produce a perfectly symmetrical waveform, which is summating, transposed on top of one another, causing a higher amplitude audio signal at a specific tone, or combination of tones that exceeds a psychological/ physiological threshold, resulting in the listener being overwhelmed with emotion." Full article here
This analysis by Patricio illustrates the unusual helical nature of Jackie's waveforms.
Theta WavesAnother theory based on waveform analysis suggests that Jackie's vibrato matches the frequency of Theta waves, human brain waves that become dominant when we feel emotional, relaxed, daydreamy or unfocused.
Here is an analysis by Luta:
"I analyzed Jackie's vibrato/tremolo with software, the frequency of her vibrato/tremolo is around 6 Hz (with subtle variations in frequency and intensity).
This is in the range of Theta brain waves (4-7.5Hz)
Theta brain waves are present during deep meditation and light sleep, including the all-important REM dream state. It is the realm of your subconsciousness, and is where you experience vivid visualizations, great inspiration, profound creativity and exceptional insight."
Examples of the Jackie Effect
Image by Ana Ausfeld
A short story exploring the Jackie Effect.
 "Memoir of Madam Jenny Lind-Goldschmidt: Her Early Art-Life and Dramatic Career, 1820-1851" ISBN 1108038689, 9781108038683
* Jackie Effect image kindly provided by "espida".