Kim Kardashian explained why she avoids smiling Monday on Instagram.

The 34-year-old reality star shared a picture of herself showing off a rare grin with family friend Shamari Maurice. Kardashian revealed she is rarely photographed smiling or laughing in order to ward off age lines.

"See I do smile... even laugh on occasion," she wrote. "Not too often though because it causes wrinkles @shammaurice."

Kardashian has over 27 million followers on Twitter, and 23 million on Instagram. She frequently posts selfies and other pictures of herself on social media, most often with a neutral expression or closed-lip smirk. The reality star has also perfected the "smize," or smiling with one's eyes.

Kardashian will release "Selfish," an entire book dedicated to her selfies, in April 2015. She and her family have starred on E! reality series "Keeping Up with the Kardashians" since October 2007, and dominated entertainment headlines in 2014.