The facts It can bend light, bring the world into focus, and next to the human brain may be our most complicated organ. But for many people the most intriguing feature of the human eye is simply its color. 

    Can it really change for no apparent reason? 

    In most people, the answer is no. Eye color fully matures in infancy and remains the same for life. But in a small percentage of adults, eye color can naturally become either noticeably darker or lighter with age. 

    What determines eye color is the pigment melanin. Eyes that have a lot of it in the connective tissue at the front of the iris, called the stroma, are darker, while those that have less tend to be lighter. 

    The levels of melanin generally remain the same throughout life, but a few things can change them permanently. 

    The first is a handful of ocular diseases like pigmentary glaucoma. Another is a condition called heterochromia, or multicolored eyes, which affects about 1 percent of the population and is often caused by traumatic injuries. An example of this can be seen in the rock star David Bowie, who attributes his contrasting eye colors, hazel and light blue, to a blow to the face as a child. 

    The third cause appears to be genetics. A study in 1997, for example, looked at thousands of twins and found that 10 percent to 15 percent of the subjects had gradual changes in eye color throughout adolescence and adulthood, which occurred at nearly identical rates in identical twins. 

    The bottom lines Eyes can change color in some people because of genetics or injury. 

    中文：

    眼睛顏色是人體最重要的特征，但科學家日前表示，人眼顏色可以改變。 

    據《紐約時報》日前報道，通常情況下，人眼在外傷作用下可能會改變顏色，但若沒有外傷的作用，眼睛顏色還會自動改變嗎？對大部分人而言，答案是否定的。眼睛顏色在人幼年時期已經形成，且基本上終生不變。但是，一小部分成年人眼睛的顏色卻可以隨著年齡的增長明顯地變深或者變淺。 

    科學家指出，決定人眼顏色的是眼球虹膜前部的基質中的黑色素，黑色素含量越多，人眼的顏色就越深，反之越淺。而基質中的黑色素含量在通常情況下是終生保持不變的，除非某些異常情況發生導致它發生永久性的改變。 

    首先，改變眼部黑色素的是眼部疾病，如青光眼（患者瞳孔內出現青綠色的反光）。 

    其次，是由外傷引起的彩色眼（又名雙眼異色癥），主要表現為雙眼顏色出現差異，如英國著名的搖滾樂手大衛·鮑伊，他的一只眼睛為淺藍色，而另一只為棕色。據悉，鮑伊曾在幼年時期因臉部受傷而導致眼睛變色。 

    第3種改變眼睛顏色的原因是遺傳基因。科學家在1997年對數千對雙胞胎研究后發現，10％至15％的雙胞胎同時在青春期和成年后眼睛顏色會逐漸改變，這證明，有相似基因的雙胞胎發生眼睛變色的幾率也是相同的。 

    科學家最后明確指出，導致人眼睛顏色變化的的關鍵就是遺傳基因和外傷。