Imagine it’s Saturday morning. You’re sipping coffee when your best friend texts, “Any chance you could help me move today?” You sigh—there go your weekend plans—but reply, “Of course.” That afternoon you sweat as you carry boxes up a flight of stairs.

想象这是一个周六早晨。你正端着咖啡小口品着，这时你最好的朋友发来短信：“今天有空帮我搬家吗？”你叹了口气——周末计划就此泡汤——但还是回复道：“当然可以。”当天下午，你满头大汗，抬着箱子爬上一段楼梯。

A week later a co-worker you barely know mentions that she’s moving and could really use a hand. This time you hesitate. You are not as quick to offer help even though the request is nearly identical.

一周后，一位你并不熟悉的同事说她要搬家，真的很需要人帮忙。这一次你犹豫了。尽管对方的请求几乎一模一样，你却没有那么快就主动提出帮忙。

Why does generosity come so naturally for those we are close to but feel more like a burden when the recipient is a stranger or mere acquaintance? Psychologists call this tendency “social discounting”: we are generally more willing to make sacrifices for people to whom we feel emotionally close, and our generosity declines as the social or emotional distance to the potential recipient of help increases.

为什么当受助者是我们亲近的人时，慷慨会显得那么自然；而当对方是陌生人或只是泛泛之交时，同样的慷慨却更像一种负担？心理学家把这种倾向称为“social discounting（社会贴现）”：一般来说，我们更愿意为在情感上与我们亲近的人做出牺牲；而随着潜在受助者与我们的社会/情感距离增大，我们的慷慨程度就会相应下降。

But what happens in the brain when we make these decisions? And why are some people more generous to socially distant individuals than others are? In recent research, my colleagues and I gained new insight into these questions by examining a rare population of individuals with selective damage to a part of the brain called the basolateral amygdala. Our findings suggest that this small but important structure may be essential for calibrating our generosity based on how close or distant others feel to us.

但当我们做出这些决定时，大脑里究竟发生了什么？为什么有些人对在情感或社会上与自己距离较远的人更慷慨？在一项近期研究中，我和同事通过考察一类罕见人群获得了新的认识：他们的大脑中一个名为“基底外侧杏仁核”（basolateral amygdala）的区域出现了选择性损伤。我们的发现表明，这个虽小却重要的结构，可能对根据我们与他人的亲疏感来“校准”我们的慷慨程度至关重要。

The amygdala, a small, almond-shaped region nestled deep in the brain’s temporal lobe, is traditionally known for its role in processing emotions, especially fear. But over the past few decades it has become clear that the amygdala, particularly its basolateral part, is a central hub in our social brain.

杏仁核（amygdala）是一块位于大脑颞叶深处、形似杏仁的小小脑区，传统上以其在情绪处理中的作用而闻名，尤其与恐惧密切相关。但在过去几十年里，研究逐渐表明：杏仁核——特别是其中的基外侧部（basolateral amygdala）——是我们“社会性大脑”的核心枢纽。

Across species, this region has been shown to participate in evaluating social rewards, empathic responses and decisions involving others. In rodents and monkeys, neurons in the basolateral amygdala encode the value of not just rewards for oneself but also the rewards received by others. And in humans, the structure has been linked to traits such as trust, empathy, moral decision-making and extraordinary altruism. Human amygdala volume also correlates with the size and complexity of a person’s social network. And some evidence suggests that psychopathy and aggression are associated with a smaller, less functional amygdala.

跨物种的研究显示，这一区域参与评估社会性奖励、移情反应以及涉及他人的决策。在啮齿动物和猴子中，基底外侧杏仁核（basolateral amygdala）的神经元不仅对自身获得的奖励价值进行编码，也会编码他人所获得的奖励价值。在人类中，该结构与信任、共情、道德决策以及非凡的利他主义等特质相关。人类杏仁核的体积还与个人社会网络的规模和复杂度相关。此外，一些证据表明，精神病态（psychopathy）和攻击性与更小、功能较弱的杏仁核有关。

So how, exactly, does the basolateral amygdala influence our decisions about whether to help others? One hypothesis is that this brain area allows us to balance competing helpful, social motives with self-interested goals. When you decide to help your best friend move, you are probably focused more on their benefit (making the move easier) than on the cost to you in time and effort. But when the person is a stranger, that mental calculation may shift. Some neuroscientists propose that the basolateral amygdala aids us as we navigate this trade-off by assigning value not just to our own well-being but also to the well-being of others.

那么，基外侧杏仁核（basolateral amygdala）究竟如何影响我们是否去帮助他人的决定呢？一种假设认为，这一脑区能让我们在“助人、合群”的动机与“自利”目标之间取得平衡。当你决定帮最好的朋友搬家时，你很可能更关注对方的收益（让搬家更轻松），而不是自己在时间和体力上的代价。但如果求助者是个陌生人，这笔心理账可能就会改变。一些神经科学家提出，基外侧杏仁核在我们权衡这种取舍时提供帮助：它不仅为我们的自身福祉赋值，也会为他人的福祉赋值。

To test this idea, my colleagues and I turned to a remarkable group of people in South Africa who have Urbach-Wiethe disease, a very rare genetic condition that causes selective bilateral damage to the basolateral amygdala while leaving the rest of the brain intact. In our study, we invited five women with this condition and 16 women without it to take part in a social discounting task. Each participant listed eight people from her own social network, ranging from her emotionally closest person (ranked as having a social distance of 1) to someone she barely knew (50) or a complete stranger (100). We then asked them to make decisions about how to split money. In each of several rounds, they received a fixed monetary amount and decided how much to share with each of their eight listed contacts. This task thus measured our participants’ willingness to share resources depending on how emotionally close or distant they felt to the people in their social network.

为检验这一设想，我和同事把目光投向了南非一组极为特殊的人群：他们患有 Urbach-Wiethe 病，这是一种极罕见的遗传性疾病，会对杏仁核的基底外侧部（basolateral amygdala）造成选择性的双侧损伤，而大脑其余部分基本完好无损。在我们的研究中，我们邀请了5名患病女性以及16名未患病的女性参与一项“社会折扣”任务。每位参与者需要从自己的社交网络中列出8个人，从情感上最亲近的人（标记为社会距离1），到几乎不认识的人（50），再到完全的陌生人（100）。随后我们请她们做出如何分配金钱的决定。在若干轮任务中，她们每轮都会获得一笔固定金额，并决定要与这8位被列出的联系人各分享多少。由此，这项任务就可以根据她们对社交网络中不同对象的情感亲近或疏远程度，来衡量其分享资源的意愿。

As expected, the participants gave more to people they were close to than they gave to others who were more distant. That is, generosity declined as social distance increased. We found it interesting, however, that participants with damage to the basolateral amygdala were less generous overall than others, and their generosity decreased more sharply as social distance increased. They showed what we call steeper social discounting: they were still willing to help those they were emotionally closest to, but their willingness to give dropped off markedly for more distant individuals.

正如预期，参与者对与自己关系亲近的人给得更多，而对社交距离更远的人给得更少。也就是说，随着社会（情感）距离增加，慷慨程度会下降。不过，我们注意到一个有趣现象：基外侧杏仁核（basolateral amygdala）受损的参与者总体上比其他人更不慷慨，而且他们的慷慨随社会距离增加而下降得更为陡峭。也就是我们所说的“更陡的社会折扣”（social discounting）：他们仍愿意帮助在情感上最亲近的人，但对更疏远的对象，给予意愿明显下滑。

One participant with basolateral amygdala damage was an exception—she was ungenerous across the board, even toward her closest friend. But overall the pattern was clear: damage to the basolateral amygdala did not eliminate altruism, although it did distort the fine-tuned calibration of generosity based on social distance.

有一位基底外侧杏仁核（basolateral amygdala）受损的参与者是个例外——她对所有人都不慷慨，甚至对自己最亲密的朋友也是如此。但总体模式很清楚：基底外侧杏仁核的损伤并不会消除利他主义，不过它确实会扭曲那种基于社会距离对慷慨程度进行的精细校准。

Notably, variations in personality, empathy or social network size did not explain the differences in generosity among our participants. Rather our participants with Urbach-Wiethe disease seemed unable to adjust their generosity flexibly to the social context.

值得注意的是，性格、共情能力或社交网络规模的差异，并不能解释我们受试者在慷慨程度上的差异。相反，患有 Urbach‑Wiethe 病（会选择性损伤基底外侧杏仁核）的受试者似乎无法根据社会情境灵活调整自己的慷慨程度。

At first glance our findings might seem to contradict earlier studies that found those with Urbach-Wiethe disease are actually more generous than others. For example, in past research people with this condition gave away more money in the trust game, a classic experiment in behavioral economics in which participants decide how much money to send to another player, the trustee. The amount sent is typically multiplied, and the trustee then decides how much to return. The initial amount sent is often seen as a measure of trust in the trustee. People with basolateral amygdala damage tend to send much more than others, even to untrustworthy trustees who fail to reciprocate.

乍一看，我们的发现似乎与早先的一些研究相矛盾——那些研究发现患有 Urbach-Wiethe disease 的人实际上比他人更慷慨。例如，在以往研究中，这类受试者在 trust game（信任博弈，这是一项行为经济学中的经典实验）里会把更多的钱交给另一名玩家，即 trustee（受托人）。被送出的金额通常会被乘数放大，随后由受托人决定返还多少。最初送出的金额常被视为对受托人的信任程度的量度。具有 basolateral amygdala（杏仁核基外侧部）损伤的人往往比他人送出更多的钱，即便对方是不会回报的不可信受托人。

Researchers have described this unusual pattern of trust as a form of “pathological altruism.” In a similar vein, the authors of one study had people with Urbach-Wiethe disease respond to moral dilemmas involving hypothetical life-or-death decisions about others. They consistently refused to sacrifice one person to save many, revealing a marked reluctance to be responsible for causing harm to another individual in comparison with participants without the disease.

研究者将这种异常的信任模式描述为一种“病理性利他主义”。同样地，一项研究的作者让患有Urbach-Wiethe病（罕见的遗传性疾病，会选择性损伤基外侧杏仁核）的人，去回应涉及对他人“生死抉择”的假想道德两难情境。他们始终拒绝为了拯救多数人而牺牲一人，显示出相较于未患病的参与者，他们明显不愿为对他人造成伤害承担责任。

How, then, can we reconcile these earlier findings with our own results? We argue that the basolateral amygdala does not simply promote or hinder prosociality. Rather it is part of a neural network that helps people create a model of how the social world works, which they then can use to guide decision-making. With an intact basolateral amygdala, a person considers social context, social structure, social norms and learned expectations in social interactions when deciding whether to be generous or selfish.

那么，我们如何把这些早期发现与我们自己的结果相协调呢？我们的观点是：基底外侧杏仁核（basolateral amygdala）并非单纯地促进或抑制亲社会性（prosociality）。相反，它是一个神经网络的一部分，这个网络帮助人们建立“社会世界如何运作”的模型，进而用来指导决策。当基底外侧杏仁核功能完好时，个体在决定是慷慨还是自利时，会在社会互动中权衡社会情境、社会结构、社会规范以及习得的期待。

When that system breaks down—as when someone suffers amygdala lesions—people may struggle to balance generous and selfish motives and consequently rely on simpler, default strategies that do not depend on networks that include this brain structure. In the trust game, the default assumption might be that others are trustworthy. In moral dilemmas, it could be to follow a rigid rule like “never harm anyone.” Such ideas might have formed in childhood and, given damage to the basolateral amygdala, not been revised later in life, even in the face of contrary experiences with untrustworthy individuals. In our task, the default strategy is to maximize one’s own payoff—unless the recipient is emotionally very close, in which case helping them comes automatically.

当这个系统失灵时——比如出现杏仁核损伤（amygdala lesions）——人们就很难在慷慨与自利的动机之间取得平衡，于是会转而依赖更简单的默认策略；这些策略并不依赖包含该脑结构的网络。在“信任博弈”（trust game）中，默认假设可能是他人是值得信任的。在道德困境中，默认可能是遵循一条僵硬的规则，比如“绝不伤害任何人”。这些观念可能在童年时期就已形成；而当外侧基底杏仁核（basolateral amygdala）受损时，即使后来在生活中反复遇到不可信之人、获得相反经验，也难以对其加以修正。在我们的任务中，默认策略是尽可能最大化自身收益——除非受助者与自己在情感上非常亲近，在那种情况下，帮助几乎会自然而然地发生。

Although our study included only a small number of participants (which was unavoidable because of the extreme rarity of the condition), the distinctive pattern of brain damage in this group—symmetrical and precisely located in both hemispheres—is quite unique in neuroscience research. Other studies involving selective brain lesions have often relied on only one or two patients. We also feel confident in our conclusions, given how our work fits into a pattern of evidence drawing from more studies and participants that suggests amygdala functionality is crucial pillar of our social life.

尽管我们的研究样本量很小（由于该疾病极为罕见，这是无法避免的），但这组受试者的脑损伤模式——在两侧大脑半球中对称、且定位精确——在神经科学研究中相当独特。其他关于选择性脑损伤的研究往往只能依赖一两名患者。并且，我们对结论仍有信心，因为我们的结果与来自更多研究和参与者的证据相吻合，这些证据共同表明：杏仁核的功能是支撑人类社会生活的关键支柱之一。

The idea that the basolateral amygdala helps us weigh selfish and altruistic motives might sound abstract, but this interaction plays out in real life all the time. Think back to the moving-day dilemma. A generous impulse to help your friend move may come automatically because it is rooted in deeply encoded values and social bonds. Yet deciding whether to help an acquaintance requires something more: flexible, model-based decision-making that weighs social norms, reputational concerns and empathy against effort costs, self-care and the simple desire to have a pleasurable, lazy weekend. It’s precisely in these gray areas that the basolateral amygdala seems to do its most important work.

认为杏仁核的基底外侧部（basolateral amygdala）帮助我们权衡自利与利他动机，这一观点也许听起来很抽象，但这种互动在现实生活中时时发生。回想前文的“搬家难题”。当对象是好友时，出手相助的慷慨冲动往往自然而然，因为它植根于深层的价值观和社会纽带。然而，是否要帮一位熟人就需要更多：一种基于模型（model-based）的灵活决策，在社会规范、名誉顾虑与同理心，和付出成本、自我照料以及想轻松度过一个慵懒周末的朴素愿望之间进行权衡。杏仁核的基底外侧部似乎正是在这些灰色地带发挥最关键的作用。

Generosity is therefore not an all-or-nothing trait; it is a model-based social behavior shaped by the people we are interacting with and how close we feel to them. And deep in the brain, the basolateral amygdala is helping us do that calculus.

因此，慷慨并不是非此即彼的特质；它是一种基于模型的社会行为，会受到我们与谁互动以及我们与对方的亲密程度所影响。大脑深处的基外侧杏仁核（basolateral amygdala）正帮助我们完成这种权衡计算。