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 intriguing¹ 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 fully² matures in infancy³ and remains⁴ the same for life. But in a small percentage of adults, eye color can naturally become either noticeably darker or lighter⁵ with age.

What determines eye color is the pigment⁶ melanin. Eyes that have a lot of it in the connective tissue at the front of the iris⁷, 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 permanently⁸.

The first is a handful of ocular diseases like pigmentary⁹ 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 adolescence¹⁰ and adulthood¹¹, 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：

　　眼睛颜色是人体最重要的特征，但科学家日前表示，人眼颜色可以改变。

　　据《纽约时报》日前报道，通常情况下，人眼在外伤作用下可能会改变颜色，但若没有外伤的作用，眼睛颜色还会自动改变吗？对大部分人而言，答案是否定的。眼睛颜色在人幼年时期已经形成，且基本上终生不变。但是，一小部分成年人眼睛的颜色却可以随着年龄的增长明显地变深或者变浅。

　　科学家指出，决定人眼颜色的是眼球虹膜前部的基质中的黑色素，黑色素含量越多，人眼的颜色就越深，反之越浅。而基质中的黑色素含量在通常情况下是终生保持不变的，除非某些异常情况发生导致它发生永久性的改变。

　　首先，改变眼部黑色素的是眼部疾病，如青光眼（患者瞳孔内出现青绿色的反光）。

　　其次，是由外伤引起的彩色眼（又名双眼异色症），主要表现为双眼颜色出现差异，如英国著名的摇滚乐手大卫·鲍伊，他的一只眼睛为浅蓝色，而另一只为棕色。据悉，鲍伊曾在幼年时期因脸部受伤而导致眼睛变色。

　　第3种改变眼睛颜色的原因是遗传基因。科学家在1997年对数千对双胞胎研究后发现，10％至15％的双胞胎同时在青春期和成年后眼睛颜色会逐渐改变，这证明，有相似基因的双胞胎发生眼睛变色的几率也是相同的。

　　科学家最后明确指出，导致人眼睛颜色变化的的关键就是遗传基因和外伤。