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. 对于全世界的生物医学研究者来说，双胞胎提供解开基因和环境，即先天与后天的影响的宝贵机会。 Because identical twins come from a single fertilized egg that splits into two, they share virtually the same genetic code. 因为同卵双生胎来自同一个受精卵，它们拥有相同的遗传密码。 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. 它们之间的任何差异肯定都源于环境因素。例如，同卵双生胎中的一个的皮肤更显年轻，这可能是由于日晒时间较短。

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. 或者，通过比较同卵双生胎和来自异卵且平均拥有其半个DNA遗传密码的异卵双生胎的经历，研究者们可以量化基因影响我们生活的程度。 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. 如果在某个小病方面，与异卵双生胎相比，同卵双生胎之间的情况如果更加相似，那么对于疾病的抵抗性至少某种程度上一定根植于遗传密码中。

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. 这两类研究——为强调环境影响而进行的同卵双生胎之间差异的研究，以及为检测遗传作用而进行的同卵双生胎和异卵双生胎之间的比较——对于理解天性和环境在决定我们的个性、行为和对疾病的抵抗力方面的相互作用是极为重要的。 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'). 使用双胞胎测量遗传影响的想法可以追溯到1875年，当时英国科学家Francis Galton第一次提出这种方法（并创造了“先天及后天“这个短语）。 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. 但是随着同卵双胞胎研究的到来，双胞胎研究在20世纪80年代出现了巨大的转折，这些同卵双胞胎是在出生时分离而在成人后再次相聚。 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. 在过去的20年里，Thomas Bouchard实验室里最终出现了137组双胞胎，他们参加了“明尼苏达州双胞胎分离研究”项目。 Numerous tests were carried out on the twins, and they were each asked more than 15,000 questions. 研究者在这里实施许多对双胞胎的测试，这些双胞胎被问及超过15,000个问题。

Bouchard and his colleagues used this mountain of data to identify how far twins were affected by their genetic makeup. Bouchard和他的同事们使用大量的数据来了解基因构成对双胞胎影响的程度。 The key to their approach was a statistical concept called heritability. 他们研究方法的关键是一个被称为“遗传”的统计学概念。 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. 从广泛意义上来讲，一个特征的遗传力可以衡量由基因差异解释群体成员间差异的程度。 And wherever Bouchard and other scientists looked, it seemed, they found the invisible hand of genetic influence helping to shape our lives. Bouchard和其他科学家好像发现基因在帮助塑造我们生活方面是一只无形的手。

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. 但是，最近双胞胎研究已经帮助科学家得出一个全新的结论：先天及后天并不是在发挥作用的唯一因素。 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. 根据表观遗传学这门最新领域，有第三种因素在起作用，在某些情况下它充当环境和基因之间的桥梁，在其他情况下它自己就可以塑造我们的个性。

Epigenetic processes are chemical reactions tied to neither nature nor nurture but representing what researchers have called a 'third component'. 表观遗传过程是一种不与先天或后天相联系的化学反应，但是它却代表研究者们所的“第三种成分”。 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. 这些反应影响我们的遗传密码的表达方式：基因如何被加强或削弱甚至开启或关闭来构成我们的骨骼、大脑以及身体的其他部位。

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. 如果把我们的DNA看作巨大的钢琴键盘、基因看作琴键的话（每个键则象征负责特定标记或性质DNA的一个片段，而所有的键合在一起则决定了我们是谁），那么表观遗传过程则决定每个键被敲击的时间和方式，以及改变被弹奏的音调。

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. 表观遗传学正改变我们对生物理解的一种方式是通过解释一种机制，通过这个机制环境直接影响基因。 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. 例如，动物研究表明：当老鼠在怀孕期经历压力时，它可能会引起胎儿的变化，那么当该啮齿动物生长时可能会出现行为问题。 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. 另外一些表观遗传过程似乎是是随机发生的，然而其他的则是正常的，例如当它们变成心脏、大脑或肺细胞时形成胚胎细胞的那些。

Geneticist Danielle Reed has worked with many twins over the years and thought deeply about what twin studies have taught us. 基因科学家Danielle Reed在过去的几年里已经对许多双胞胎进行研究，并且对双胞胎研究所告知我们的内容进行了深入思考。 'It's very clear when you look at twins that much of what they share is hardwired,' she says. “当你看着双胞胎时，很明显会感到他们很多共性都是天生内在的”她说。 'Many things about them are absolutely the same and unalterable. “关于他们的许多事情都是绝对相同的并且无法更改。 But it's also clear, when you get to know them, that other things about them are different. 但是当你了解他们后，你会很明显感到他们在其他方面是不同的的。 Epigenetics is the origin of a lot of those differences, in my view.' 在我看来，表观遗传学是许多差异的起源。”

Reed credits Thomas Bouchard's work for today's surge in twin studies. Reed将今天对双胞胎研究的出色结果归功于Thomas Bouchard的工作。 'He was the trailblazer,' she says. “他是先驱，”’她说， 'We forget that 50 years ago things like heart disease were thought to be caused entirely by lifestyle. “我们可能会忘记50年前，人们认为心脏病这之类的疾病完全是由生活方式引起的。 Schizophrenia was thought to be due to poor mothering. 精神分裂者是由于较差的抚养导致的。 Twin studies have allowed us to be more reflective about what people are actually born with and what's caused by experience.' 双胞胎研究已经让我们更加对人类生来实际是什么以及经历所带来的变化进行思考。”

Having said that, Reed adds, the latest work in epigenetics promises to take our understanding even further. 在此基础上，Reed补充道，对表观遗传学的最新研究将使我们的理解更加深刻。 'What I like to say is that nature writes some things in pencil and some things in pen,' she says. “我想要说的是天性使用铅笔和钢笔都写出了某些事，”她说， 'Things written in pen you can't change. “使用钢笔书写的内容无法改变。 That's DNA. 因为那是DNA。 But things written in pencil you can. 但是使用铅笔书写的内容你却可以改变。 That's epigenetics. 那是表观遗传。 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.' 事实上现在我们可以研究DNA并查看到铅笔书写的位置，它将成为某种意义上的新世界。”