362 J. Cosmet. Sci., 73, 362–376 (November/December 2022) Address all correspondence to Ken Nishino, Nishino.ken@kao.com Differential Effects of Eyeliner Color and Thickness Applications on the Perceived Color of the Eye Sclera KEN NISHINO AND NATSUKO OKIYAMA Makeup Product Research Laboratory, Kao Corporation, Odawara, Japan (K.N., N.O.) Accepted for publication February 14, 2023. Synopsis The most widely studied influence of eye makeup on perception is the eye enlargement effect. There are limited studies on other influences on visual perception. This study aimed to demonstrate the influence of eyeliner color and thickness on the perceptual color of sclera, the white part of the eyes, and to understand the relationship between them. A psychophysical experimental method, the staircase method, was employed to evaluate the influence of eyeliners on perception. The first experiment showed that thicker eyeliners resulted in perceptually larger eyes and a white sclera. The second experiment revealed that eyeliner colors that were significantly different from skin color, such as blue and green, made the perceptual sclera color whiter. The third experiment confirmed that the hue of the eyeliner color considerably affected perceptual sclera whiteness, whereas the lightness of the eyeliner affected perceptual eye size. An influence on sclera whiteness perception by cosmetic eyeliners was demonstrated, and the relationship between the influence on perception and the characteristics of the applied eyeliner was clarified. This effect may be one of the mechanisms behind the impressive eyes obtained by eye makeup, and the results will provide important scientific evidence for improving makeup products and techniques. INTRODUCTION Cosmetics such as foundations, lip rouges, and eye makeup can notably enhance female facial attractiveness by changing the appearance of facial features in several ways. In particular, eye makeup products such as mascaras, eyeliners, and eye shadows significantly influence facial attractiveness (1). Among the various beautification mechanisms, the perceptual eye enlargement effect is the most widely studied effect. Childlike features such as full lips and large eyes are considered attractive (2). Matsushita et al. quantitatively demonstrated that all three eye makeup products cited above perceptually enlarged eye size by approximately 5% (3). Other mechanisms may also influence attractiveness, but they have rarely been studied (4). Although various products have been created based on sensory evaluations, advances in mechanistic understanding can help develop more effective products. Studies on the basic mechanism for determining a new phenomenon are limited. Detailed analyses and research in the fields of vision, optics, and physics are important for the development of product design.