How poverty passes from generation to generation is now becoming clearer. The answer lies in the effect of stress on two particular parts of the brain

HAT the children of the poor underachieve in later life, and thus remain poor themselves, is one of the enduring problems of society. Sociologists have studied and described it. Socialists¹ have tried to abolish it by dictatorship and central planning. Liberals have preferred democracy and opportunity. But nobody has truly understood what causes it. Until, perhaps, now.

The crucial breakthrough was made three years ago, when Martha Farah of the University of Pennsylvania showed that the working memories of children who have been raised in poverty have smaller capacities than those of middle-class children. Working memory is the ability to hold bits of information in the brain for current use—the digits² of a phone number, for example. It is crucial for comprehending languages, for reading and for solving problems. Entry into the working memory is also a prerequisite³ for something to be learnt permanently⁴ as part of declarative memory—the stuff a person knows explicitly⁵, like the dates of famous battles, rather than what he knows implicitly⁶, like how to ride a bicycle.

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Since Dr Farah’s discovery, Gary Evans and Michelle Schamberg of Cornell University have studied the phenomenon in more detail. As they report in this week’s Proceedings⁷ of the National Academy of Sciences, they have found that the reduced capacity of the memories of the poor is almost certainly the result of stress affecting the way that childish brains develop.

Dr Evans’s and Dr Schamberg’s volunteers were 195 participants in a long-term sociological and medical study that Dr Evans is carrying out in New York state. At the time, the participants were 17 years old. All are white, and the numbers of men and women are about equal.

Stress in the city

To measure the amount of stress an individual had suffered over the course of his life, the two researchers used an index known as allostatic load. This is a combination of the values of six variables: diastolic and systolic blood pressure; the concentrations of three stress-related hormones⁸; and the body-mass index, a measure of obesity⁹. For all six, a higher value indicates a more stressful life; and for all six, the values were higher, on average, in poor children than in those who were middle class. Moreover, because Dr Evans’s wider study had followed the participants from birth, the two researchers were able to estimate what proportion of each child’s life had been spent in poverty. That more precise figure, too, was correlated with the allostatic load.

The capacity of a 17-year-old’s working memory was also correlated with allostatic load. Those who had spent their whole lives in poverty could hold an average of 8.5 items in their memory at any time. Those brought up in a middle-class family could manage 9.4, and those whose economic and social experiences had been mixed were in the middle.

These two correlations¹⁰ do not by themselves prove that chronic¹¹ stress damages the memory, but Dr Evans and Dr Schamberg then applied¹² a statistical¹³ technique called hierarchical regression to the results. They were able to use this to remove the effect of allostatic load on the relationship between poverty and memory discovered originally by Dr Farah. When they did so, that relationship disappeared. In other words, the diminution¹⁴ of memory in the poorer members of their study was entirely¹⁵ explained by stress, rather than by any more general aspect of poverty.

To confirm this result, the researchers also looked at characteristics such as each participant’s birthweight, his mother’s age when she gave birth, the mother’s level of education and her marital¹⁶ status, all of which differ, on average, between the poor and the middle classes. None of these characteristics had any effect. Nor did a mother’s own stress levels.

That stress, and stress alone, is responsible for damaging the working memories of poor children thus looks like a strong hypothesis. It is also backed up by work done on both people and laboratory animals, which shows that stress changes the activity of neurotransmitters, the chemicals that carry signals from one nerve cell to another in the brain. Stress also suppresses the generation of new nerve cells in the brain, and causes the “remodelling” of existing ones. Most significantly of all, it shrinks the volume of the prefrontal cortex and the hippocampus. These are the parts of the brain most closely associated with working memory.

Children with stressed lives, then, find it harder to learn. Put pejoratively¹⁸, they are stupider. It is not surprising that they do less well at school, end up poor as adults and often visit the same circumstances on their own children.

Dr Evans’s and Dr Schamberg’s study does not examine the nature of the stress that the children of the poor are exposed to, but it is now well established that poor adults live stressful lives, and not just for the obvious reason that poverty brings uncertainty¹⁹ about the future. The main reason poor people are stressed is that they are at the bottom of the social heap as well as the financial one.

Sir Michael Marmot, of University College London, and his intellectual successors have shown repeatedly that people at the bottom of social hierarchies²⁰ experience much more stress in their daily lives than those at the top—and suffer the consequences in their health. Even quite young children are socially sensitive beings and aware of such things.

So, it may not be necessary to look any further than their place in the pecking order to explain what Dr Evans and Dr Schamberg have discovered in their research into the children of the poor. The Bible says, “the poor you will always have with you.” Dr Evans and Dr Schamberg may have provided an important part of the explanation why.

贫困是怎样代代相传的，现在越来越清楚了。答案就在于压力对大脑两个部分的影响。

儿童期遭遇贫困的人，在以后的生活表现不佳，从而继续保持贫困，这是一个持续的社会问题。社会学家们对此进行了大量的研究和描述。为了根除这一现象，社会主义曾试图采取集权和计划，而自由主义则希望通过民主与机会。但也许没有人真正了解究竟原因导致了这一现象。

三年前，这个问题取得到了突破性的进展。宾夕法尼亚大学的Martha Farah 表明，贫困儿童的工作记忆体相比出生中产阶级的儿童要小。工作记忆是能够在大脑中把握信息而用于现实的能力，就像从头脑中调出数字打电话一样。它对语言的理解，阅读和解决问题都至关重要。进入工作记忆也是一个先决条件，它是一种将明确认知变为大脑永久记忆部分的能力，如毫不含蓄地记住著名战役的日期，骑自行车等。

由于Farah博士的这一发现，康奈尔大学的Gary Evans 和Michelle Schamberg 对此进行了更详细的研究。正如他们在本周美国国家科学院的报告，他们发现，由于贫而致的记忆能力下降几乎可以肯定是影响儿童大脑发展的原因。

Evans博士和Schamberg 博士对195名志愿者进行了长期的社会跟踪和医学研究，具体由Evans博士负责在纽约州开展，参与者都17岁，全部白种人，男性和女性人数大约相等。

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这两位研究人员使用了称为适应负荷的方法来衡量一个人在遭受压力过程中显出的各项指数。包含六个变量：舒张压和收缩压;三种与荷尔蒙有关的浓度，体质指数，肥瘦程度。数值越高显示承受越多的生活压力。研究表明，平均而言贫困儿童的各项指数要高于那些中产阶级儿童。此外，由于Evans博士对参与者从出生开始进行了更广泛地跟踪研究，使得两个研究人员能够依据每个儿童在贫困中度过的时间长短进行衡量，从而更准确地描绘了以此相关的负荷状况。

一个17岁大的工作记忆状况同样也与负荷相关。那些一直生活在贫困中的人，记忆体的容量一直在8.5 左右，而生活在中产家庭的则可以达到9.4。中间缺失的部分，就是经济的和社会的混合经验。

这种相关性本身，并不能证明长期的压力会损害记忆力。于是两位研究者应用分层回溯的统计技术对结果再次研究，以求移除负荷压力在贫困和记忆二者之间的影响，也就是Farah博士的研究发现。但当他们这样做时，这种关系消失了。换言之，压力是导致较贫穷成员记忆体缩小到关键原因，而非其他。

为了确认这一结果，研究人员还考察了一些特点，诸如每个参加者的体重，，出生时母亲的年龄，母亲的受教育程度和她的婚姻状况，所有这一切都尽相同，包括母亲受压力的程度。平均而言，这些特点对无论穷人还是中产阶级的工作记忆都没有任何影响。

压力，只有紧张的压力，对贫困儿童工作记忆体的损害负有责任。这看似一个强有力的前提。同时也在对人研究和实验动物两个方面得到了验证。这表明，压力会改变神经递质的化学特性，即影响信号从一个神经细胞到另一个神经细胞的传递。还会抑制新的神经细胞在大脑中的产生，并导致现有大脑的“重塑”。最为明显的是，它缩小了前额叶皮质和海马体的体积。而这些地区恰恰与大脑的工作记忆密切相关。

这些面对生活压力的儿童，学习往往不佳，被认为比较笨。也就不奇怪他们最后长大成人会变为穷人，同时让他们的子女遭遇相同的境况。

Evans博士和Schamberg 博士的研究没有涉及贫困儿童面临压力的性质，但有一点是确定的，成年人贫困的生活，会导致未来的不确定性。不论经济状况还是社会地位穷人都处于最低层，这是压力的主要原因。

伦敦大学学院的迈克尔爵士用他的智慧再三表明，相对于社会底层民众每天所面对的生活压力，上层人物对的健康担忧显得像是无病呻吟。即使是非常年幼的儿童，都能敏感并意识到这点。

因此，也许没有必要对Evans博士和Schamberg 博士的研究发现咬文嚼字。圣经说， “穷人，你将常伴穷困 ”。也许，他们提供了一个重要的解释。