other approaches are also shedding light on the question. in 2000, andreas bartels and semir zeki of university college, london, located the areas of the brain activated by romantic love. they took students who said they were madly in love, put them into a brain scanner, and looked at their patterns of brain activity.the results were surprising. for a start, a relatively small area of the human brain is active in love, compared with that involved in, say, ordinary friendship. “it is fascinating to reflect”, the pair conclude, “that the face that launched a thousand ships should have done so through such a limited expanse of cortex.” the second surprise was that the brain areas active in love are different from the areas activated in other emotional states, such as fear and anger. parts of the brain that are love-bitten include the one responsible for gut feelings, and the ones which generate the euphoria induced by drugs such as cocaine. so the brains of people deeply in love do not look like those of people experiencing strong emotions, but instead like those of people snorting coke. love, in other words, uses the neural mechanisms that are activated during the process of addiction. “we are literally addicted to love,” dr young observes. like the prairie voles.
it seems possible, then, that animals which form strong social bonds do so because of the location of their receptors for vasopressin and oxytocin. evolution acts on the distribution of these receptors to generate social or non-social versions of a vole. the more receptors located in regions associated with reward, the more rewarding social interactions become. social groups, and society itself, rely ultimately on these receptors. but for evolution to be able to act, there must be individual variation between mice, and between men. and this has interesting st year, steven phelps, who works at emory with dr young, found great diversity in the distribution of vasopressin receptors between individual prairie voles. he suggests that this variation contributes to individual differences in social behaviour—in other words, some voles will be more faithful than others. meanwhile, dr young says that he and his colleagues have found a lot of variation in the vasopressin-receptor gene in humans. “we may be able to do things like look at their gene sequence, look at their promoter sequence, to genotype people and correlate that with their fidelity,” he muses.
it has already proved possible to tinker with this genetic inheritance, with startling results. scientists can increase the expression of the relevant receptors in prairie voles, and thus strengthen the animals' ability to attach to partners. and in 1999, dr young led a team that took the prairie-vole receptor gene and inserted it into an ordinary (and therefore promiscuous) mouse. the transgenic mouse thus created was much more sociable to its mate.
虽然抗利尿激素和催产素在人脑内的角色仍未被透彻了解，但与田鼠一样，人体在受到性刺激时也会释放这些荷尔蒙。尽管与田鼠不一样，人类不太可能有一张基于嗅觉的神经系统伴侣图谱，但若干线索已有力说明，这两种荷尔蒙能揭示一些人类恋爱的本质∶在已被研究过的人类灵长目同伴之中，一夫一妻制小长尾猴的大脑奖赏中枢系统中，抗利尿激素水平较非一夫一妻制的恒河短尾猿高。其他方法也正在为人们寻找这些问题的谜底。2000 年，来自伦敦市大学学院的andreas bartels和semir zeki定位出了能被浪漫爱情激活的大脑区域。两位学者选择自称正在热恋的学生作为测试目标，利用脑扫描仪对他们的大脑活动模式进行观测。
令人惊讶的是：首先，人脑参与到恋爱的活动区域，较之其他感情(如普通友谊)，相对要比较小些。“引人注意的是，结果显示，”两位学者推断说，“美丽的面容是通过控制一个有限区域的大脑皮层来实现‘一顾倾人城，再顾倾人国’的。” 第二个惊奇之处是，大脑内因恋爱而活跃的区域不同于因其他情绪而活跃的区域，例如，恐惧和愤怒。 被恋爱“咬住”的那部份大脑还包括负责内脏感觉和因可卡因等毒品生成快感的区域。因此，因此，深坠爱河的恋人们的大脑，并非类同于经历强烈情绪波动的人，倒更接近那些鼻吸可卡因的瘾君子。换句话说，爱情使用的是在成瘾过程中被激活的神经机制。“严格地讲, 我们成瘾于爱情，”young博士评述道，“就如同草原田鼠”。那么看起来，能形成稳固社会联结的动物之所以愿意建立彼此间的伙伴关系，可能是由它们的抗利尿激素和催产素受体在大脑中的位置所决定。生物进化作用于以上受体在大脑的分布，而产生了社会性或非社会性不同版本的田鼠。位于奖赏机制有关区域的受体越多，对个体而言, 社会互动就更具有回报性。动物的社会性团体，乃至整个社会本身都最终依赖于这些受体。但要使进化真能发生效力，在鼠之鼠之间以及人与人之间，一定存在个体变异。这一论述还有引人入胜的推论。
在emory大学与young博士一同工作的steven phelps去年发现，草原田鼠各个体间抗利尿激素受体的脑内分布，存在着很大的差异。他提出，正是这一变异导致了社会行为的个体差异，换句话说，一些田鼠将会比另一些更加忠诚。同时，young博士说他和同事已经发现人类抗利尿激素受体基因的许多变异。“我们或许能够做些类似于察看人们的基因序列，察看他们的启动序列等工作，在此基础上对人们进行基因型分类, 并把分类结果与他们的忠诚度关联起来”，young博士作如是想。
事实已经证明可能对这种基因遗传进行修补，并产生了令人吃惊的结果。科学家能增加草原田鼠相关受体的表达，以加强动物对同伴的依附能力。而且在 1999 年，young博士带领了一只研究队伍，他们将草原田鼠的受体基因插入到一只平常(因此成为杂乱的) 老鼠的体内，由此产生的转基因鼠对它的配偶表现得更为友善。