对于全世界的生物医药学研究者来说，双胞胎提供了一个宝贵的机会以供他们探究基因和环境——也就是先天和后天一所产生的影响。因为同卵双胞胎来自于分裂为二的同一个受精卵，所以他们实际上享有着完全相同的基因代码。二人之间的任何差异一例如双胞胎中的某个有着看起来更为年轻的肌肤——都必定是环境因素造成的，比如日晒时间更少。

To biomedical researchers all over the world, twins offer a precious opportunity to untangle the influence of genes and the environment - of nature and nurture. Because identical twins come from a single fertilized egg that splits into two, they share virtually the same genetic code. Any differences between them - one twin having younger looking skin, for example - must be due to environmental factors such as less time spent in the sun.

另一方面，通过比较同卵双胞胎与异卵双胞胎(后者来自两个不同的受精卵而拥有平均来说约为一半的相同基因)的经历，研究人员就可以量化地得出我们的基因到底在何种程度上影响着我们的生活。如果对于某种疾病同卵双胞胎二人之间的反应比起异卵双胞胎来更为相似，那么容易得上这种疾病的特征就至少有一部分原因是来自遗传因素。

这样两条研究线索——研究同卵双胞胎之间的不同以确认环境的影响，以及比较同卵双胞胎与异卵双胞胎之间的异同以衡量遗传因素所扮演的角色——一直以来都是至关重要的，我们需要借此来理解先天因素与后天因素是如何综合作用起来決定我们的个性、行为和易感染某些疾病的程度。

Alternatively, by comparing the experiences of identical twins with those of fraternal twins, who come from separate eggs and share on average half their DNA, researchers can quantify the extent to which our genes affect our lives. If identical twins are more similar to each other with respect to an ailment than fraternal twins are, then vulnerability to the disease must be rooted at least in part in heredity.

研究双胞胎以衡量遗传因素的影响这个理念可以追溯到1875年，当时英国科学家Francis Gallon第一次提出了这样一种方法(并且创造了“先天和后天”这样一种说法)。但是双胞胎研究在20世纪80年代迎来了一个令人意想不到的转折，当时出现了这样一种思路，其研究对象为那些出生时即被分开而各自长大成人后才重新聚首的同卵双胞胎们。在二十年的时间里，前后共有137对双胞胎走进了Thomas Bouchard的实验室，这次研究后来成为了广为人知的“被异地养大双胞胎的明尼苏达研究”。在这些双胞胎身上开展了不计其数的测试，他们中的每个人都回答了15,000多个问题。

Bouchard和他的同事们利用了这批数目惊人的海量数据来辨识双胞胎到底在何种程度上受到其基因组成的影响。他们所采用的研究方法的关键在于一种称之为“可遗传性”的统计学数理概念。从广义概念上来说，任何某种特质的可遗传性所衡量的是人口群体中各个成员之间的个体差异可以在何种程度上由他们彼此之间基因方面的差异来解释。而似乎无论Bouchard和其他科学家们如何看待这个问题，他们都能发现基因影响力的这只无形之手在参与塑造我们的人生。

These two lines of research - studying the differences between identical twins to pinpoint the influence of environment, and comparing identical twins with fraternal ones to measure the role of inheritance - have been crucial to understanding the interplay of nature and nurture in determining our personalities, behavior, and vulnerability to disease.

然而，近年来，对于双胞胎的研究已帮助引导科学家们得出了一个不同于以往的新结论：并非只有先天和后天这两个基础原因在发生作用。根据一个近期出现的被称为“表观遗传性”(epigenetics)的研究新领域的看法，还有第三个因素在起作用，它有时候作为一道连接桥梁，贯通于环境与我们自身基因的二者之间；而另一些时候则直接着手塑造我们每一个人。

表观遗传的过程是这样一些化学反应，它们既不与先天也不与后天相关，而是代表着研究人员所称之为的“第三组成因素”。这些反应影响着我们基因代码的表现方式：每一项基因是如何得到加强或削弱，甚至是被激活或关闭，从而构建起我们的骨骼、大脑和身体的所有其他组成部分。

The idea of using twins to measure the influence of heredity dates back to 1875, when the English scientist Francis Galton first suggested the approach (and coined the phrase 'nature and nurture'). But twin studies took a surprising twist in the 1980s, with the arrival of studies into identical twins who had been separated at birth and reunited as adults. Over two decades 137 sets of twins eventually visited Thomas Bouchard's lab in what became known as the Minnesota Study of Twins Reared Apart. Numerous tests were carried out on the twins, and they were each asked more than 15,000 questions.

如果你把我们的DNA设想成一组巨大的钢琴键盘而我们的基因就是其中的琴键——每个琴键象征着DNA的一个片段，负责某个音调或者说特质，而所有的琴键组合起来就构成了每一个独特的我们，那么表观遗传的过程就决定着每一个琴键可以什么时候、以何种方式被弹响，从而改变着演奏曲目的旋律。

表观遗传学研究之所以彻底改变了我们对于生物学的理解，方式之一就在于它揭示出这样一种机制，外在环境正是通过这样一种机制直接作用于内在基因。举例来说，动物研究已经证实了：当一只老鼠在怀孕期间有过紧张压力的体验，就可能在其胎儿中引发表观遗传性的改变，进而随着这只啮齿动物的成长导致其行为方面的各种问题。有一些表观遗传过程似乎是随机发生的，而另一些则为常规现象，例如那些指导胚胎细胞如何分化成为心脏、大脑或肝脏细胞的过程。

Bouchard and his colleagues used this mountain of data to identify how far twins were affected by their genetic makeup. The key to their approach was a statistical concept called heritability. In broad terms, the heritability of a trait measures the extent to which differences among members of a population can be explained by differences in their genetics. And wherever Bouchard and other scientists looked, it seemed, they found the invisible hand of genetic influence helping to shape our lives.

基因学家Danielle Reed多年来研究过许多对双胞胎，深入思考过双胞胎研究到底可以教会我们哪些知识。“当观察一对双胞胎时，你能够清楚地看出他们之间所共有的许多东西都是基因硬件所決定的，”她这样说道。“关于他们的许多东西都是绝对一模一样和不可改变的。但是当你深入了解了他们以后，同样清楚显现的则是他们身上还有很多其他东西都是不一样的。在我看来，表观遗传学正是大部分这些差异之所以产生的根源所在。”

Reed认为今天关于双胞胎研究的蓬勃发展都要归功于Thomas Bouchard的工作。“他是这个领域的先驱，”她说到。“我们忘记了，就在50年前，例如心脏疾病这样的事情还被认为完全是由个人生活方式引起的。精神分裂症过去曾被视为原因在于母亲养育得不好。双胞胎研究使得我们去进一步深思人们到底生而具有哪些特征，又有哪些情況是由个人经历所造成的。”

Lately, however, twin studies have helped lead scientists to a radical new conclusion: that nature and nurture are not the only elemental forces at work. According to a recent field called epigenetics, there is a third factor also in play, one that in some cases serves as a bridge between the environment and our genes, and in others operates on its own to shape who we are.

说完这些，Reed还补充说，最近在表观遗传学领域中的研究工作还很有可能带领我们的理解更进一步。“我想要说的是，我们的先天用铅笔写好了一些内容，又用墨笔写好了另一些内容，”她这样说。“用墨笔写下的内容是无法改变的，那是我们的DNA。但是用铅笔写下的内容却可以修改，那就是表观遗传学。现在既然我们己经有能力探査DNA并看出哪些是铅笔写出来的内容了，这便进入了一个全新的世界。”

Epigenetic processes are chemical reactions tied to neither nature nor nurture but representing what researchers have called a 'third component'. These reactions influence how our genetic code is expressed: how each gene is strengthened or weakened, even turned on or off, to build our bones, brains and all the other parts of our bodies.

If you think of our DNA as an immense piano keyboard and our genes as the keys - each key symbolizing a segment of DNA responsible for a particular note, or trait, and all the keys combining to make us who we are - then epigenetic processes determine when and how each key can be struck, changing the tune being played.

One way the study of epigenetics is revolutionizing our understanding of biology is by revealing a mechanism by which the environment directly impacts on genes. Studies of animals, for example, have shown that when a rat experiences stress during pregnancy, it can cause epigenetic changes in a fetus that lead to behavioral problems as the rodent grows up. Other epigenetic processes appear to occur randomly, while others are normal, such as those that guide embryonic cells as they become heart, brain, or liver cells for example.

Geneticist Danielle Reed has worked with many twins over the years and thought deeply about what twin studies have taught us. 'It's very clear when you look at twins that much of what they share is hardwired 'she says. 'Many things about them are absolutely the same and unalterable. But it's also clear, when you get to know them, that other things about them are different. Epigenetics is the origin of a lot of those differences, in my view.'

Reed credits Thomas Bouchard's work for today's surge in twin studies. 'He was the trailblazer,' she says. 'We forget that 50 years ago things like heart disease were thought to be caused entirely by lifestyle. Schizophrenia was thought to be due to poor mothering. Twin studies have allowed us to be more reflective about what people are actually born with and what's caused by experience.'

Having said that, Reed adds, the latest work in epigenetics promises to take our understanding even further. 'What I like to say is that nature writes some things in pencil and some things in pen 'she says. 'Things written in pen you can't change. That's DNA. But things written in pencil you can. That's epigenetics. Now that we're actually able to look at the DNA and see where the pencil writings are, it's sort of a whole new world.'