"Portrait of a Young Fiancée," which is thought to be of an Italian girl named Bianca, shows the same almost-smile depending on viewing distance and sharpness of the image.
"When she is viewed from close-up the mouth appears to have a downwards slant, making her look melancholic, unhappy and hostile, but when viewed from further away, her mouth appears to take an upward, smile-like appearance making her appear happy and cheerful," researchers wrote.
Click through for more images of the iconic painting:
The optical illusion hidden in the 'Mona Lisa' explained
A man views the painting "Portrait of Lisa Gherardini del Giocondo", also known as Mona Lisa, and painted by an assistant of Italian Renaissance master Leonardo da Vinci, at the Louvre Museum in Paris, Tuesday, March 27, 2012. The painting is owned by the Prado museum of Madrid, and was painted around 1503. An Da Vinci exhibition starts on Thursday with the unfinished artpiece "The Virgin and Child with Saint Anne" as the star of a major exhibit exploring the work's genesis, and its place in art history. (AP Photo/Jacques Brinon)
Members of the media are gathered next to the Mona Lisa, during an event to unveil the new lighting of Leonardo da Vinci's painting Mona Lisa, also known as La Joconde, at the Louvre museum in Paris, Tuesday June 4, 2013. Mona Lisa is now illuminated by LED lighting. The lighting had to meet various technical specifications, but also meet the more subjective and aesthetic requirements of the museum Director and Franceâs Historical Monuments Committee.(AP Photo/Remy de la Mauviniere)
FILE - This Monday, April 26, 2004 file photo shows a man taking a photograph of Leonardo da Vinci's 16th century masterpiece the "Mona Lisa" painting, kept behind a protective glass, in the Louvre museum in Paris. Italian researchers have announced, Tuesday, April 5, 2011, a plan to dig up bones in a Florence convent in hopes of identifying the remains of a Renaissance woman long believed to be the model for the Mona Lisa. The researchers hope that the project can help answer some of the enduring mysteries surrounding Leonardo da Vinci's masterpiece, including whether the woman, Lisa Gherardini, was indeed the model. The excavations in the Convent of St. Ursula, in central Florence, are scheduled to begin at the end of April. (AP Photo/Amel Pain, File) IMAGE MUST BE USED IN ITS ENTIRITY
In this photo released by the Louvre Museum, people examine the painting Mona Lisa by Leonardo Da Vinci during its moving in Paris Monday April 4, 2005. The Louvre Museum has moved Da Vinci's ``Mona Lisa'' to a bigger room, refurbished at a cost of euro 4.8 million (US$ 6.1 million), giving a better view of the 500-year-old masterpiece to the millions who come to see it every year. (AP Photo/Le Louvre, P Ballif)
ARCHIV: People watch the Mona Lisa painting by Leonardo Da Vinci, left, at its new place in the Louvre museum, in Paris (Foto vom 05.04.05). Erst der Diebstahl vor 100 Jahren hat dem legendaeren Bild des Renaissance-Malers Leonardo da Vinci, "Mona Lisa", nach Ansicht des Leipziger Kunsthistorikers Frank Zoellner zu Weltruhm verholfen. Noch im 19. Jahrhundert sei Leonardos Abendmahl deutlich beliebter und beruehmter gewesen, sagte der Wissenschaftler im dapd-Interview. (zu dapd-Text) Foto: Francois Mori/AP/dapd
FILE - In this April 5, 2005 file photo, a woman watches the painting 'Mona Lisa' by Leonardo Da Vinci in the Louvre Museum in Paris. The Louvre Museum says Tuesday Aug.11, 2009 a Russian visitor hurled an empty terra cotta mug at the Da Vinci masterpiece on Aug.2, 2009. The canvas was undamaged, a museum spokesman says, though the mug shattered. (AP Photo/Francois Mori; files)
PARIS, FRANCE - FEBRUARY 28: Visitors take pictures of Leonardo da Vinci 'Mona Lisa' inside the Louvre museum on February 28, 2014 in Paris, France. The museum is housed in the Louvre Palace, one of the world's largest museums which opened 1793. (Photo by Christian Marquardt/Getty Images)
Japanese Prime Minister Shinzo Abe (Front) and his wife Akie (3rd R) look at 'La Joconde', a 1503-1506 oil on wood portrait of Mona Lisa by Leonardo Da Vinci, at the Louvre Museum in Paris on May 4, 2014. Japanese Prime Minister Shinzo Abe arrived in France Sunday on the latest leg of a six-nation European tour for trade and security talks at a time of mounting tensions with China. Abe arrived in the French capital on Sunday afternoon and immediately left for a private visit to the Louvre that included stops at the museum's best known works, including the Mona Lisa and Venus de Milo. AFP PHOTO / STEPHANE DE SAKUTIN (Photo credit should read STEPHANE DE SAKUTIN/AFP/Getty Images)