有“双性恋基因”的直男有更多孩子？性行为研究可以有多不靠谱｜科学60秒

2024-03-23 14:03

性行为的遗传学研究，能有多不靠谱？

我们与基因的关系是复杂的，DNA 编码在某种意义上决定了我们是谁，但我们又不完全由基因决定。最近，一些科学家就研究了所谓的“双性恋基因”，和这些基因与爱冒险性格的关系。

这项研究在互联网上已经掀起了一波热潮，许多媒体以非常吸引眼球的标题报道了这项研究，比如“突破性新研究表明，双性恋基因有助于防止同性恋者灭绝”、“研究发现，有‘双性恋基因’的直男有更多孩子”、“新研究称双性恋者更爱冒险，我们询问了双性恋者这是否属实”……

当然，这些表述夸张了一些。研究人员使用了英国生物样本库（UK Biobank）数据库中一个 2006 年启动的子数据集，用全基因组关联分析（GWAS）技术分析了其中 45 万英国人的基因数据和自我报告信息。这些人全部是白人男性，且在参与英国生物样本库调查时年龄在 40~69 岁之间，也就是说，该研究仅适用于这一非常特定的人群。

研究人员分析了这些人的基因，并交叉比对了他们对问卷中某些问题的回答，发现基因和男性（仅限男性）双性恋行为存在某种联系，并且，男性双性恋行为和仅同性恋行为相关联的基因完全不同。须要明确的是，这些研究只是发现了基因和双性恋行为的相关性，是否有双性恋行为也是由受调查者自行填写的。

研究还包含了一些颇有争议的发现，例如，有“双性恋行为相关基因”的男性也具有冒险倾向相关基因。要注意的是，研究中对冒险倾向的判断仅基于问卷里的一个“是/否”问题：“你是否认为自己是一个喜欢冒险的人？”另一个发现则是，在拥有“双性恋行为相关基因”但是说自己只存在异性行为的男性中，这些“双性恋基因”和冒险基因还和自我报告的性伴侣数量较多相关。研究人员由此推测，这可能解释了为什么“双性恋基因”和双性恋行为在人类演化过程中一直存在，也就是说，这些基因可能会让直男有更多的性生活和更多的后代。研究的通讯作者、美国密歇根大学（University of Michigan）教授张建之表示：“基本的发现是双性恋行为和子女数量在遗传上呈正相关。”

论文作者做了很多大胆的假设，才最终得出了这个结论。美国耶鲁大学（Yale University）的遗传学助理教授史蒂文·赖利（Steven Reilly）认为，在执行和解释这个研究的时候应更加谨慎。乔安娜·魏斯特（Joanna Wuest）是美国蒙特霍利约克学院（Mount Holyoke College）的政治学助理教授，她表示：“这是基于少量的数据讲了大量的故事。”

从这项研究出发，很多批评的声音指向了近些年涌现的用 GWAS 分析人类性行为的研究。一个重要的例子是 2019 年的一项研究，它指出遗传因素仅解释了性取向的 8%~25%。这个研究是对已被证伪的“同性恋基因”假设的进一步反驳，它表明性取向是一系列复杂因素的结果，基因、环境、社会和文化因素都有一定的影响。

当我们再谨慎地看 2019 年的那项研究时，会发现它最大的问题还是：GWAS 只能揭示相关性，不能证明因果。美国普林斯顿大学（Princeton University）的人类学教授奥古斯丁·富恩特斯（Agustin Fuentes）也表示：“GWAS 能告诉你的事情远不如一些人想的那么多，它们不能告诉你任何的原因或结果。”并且，GWAS 研究指出的相关性也可能根本不是科学家认为自己正在研究的东西。

人类是复杂的生物，人的自我认同、性格特点、行为特质都难以定义。在遗传学研究中，我们给人类的特质人为划分了各种类别，但正如富恩特斯所说，双性恋并不一定意味着一个人要和有阴茎的人、有阴道的人发生过性关系。然而，这项新研究和其他类似的研究就是如此定义的双性恋行为，并且是基于人们自己回答的性经历。

这项研究对象是一群年纪较大的男性，在他们的年轻时代，同性行为仍然有部分社会污名。那时候的社会环境可能阻止了一些人进行同性行为。此外，人们不一定如实和科学家分享了自己的性经历，人们回答问题时可以有意无意地撒谎或者忽视掉一些事情。“性”本身也是主观的，一个人定义下的性行为……[查看全文]

What Do You Mean, Bisexual People Are 'Risk-Taking'? Why Genetic Studies about Sexuality Can Be Fraught

Lauren Leffer: The link between our genes and our selves is complicated. We know the DNA coded in our cells is part of what makes each of us who we are.

Tulika Bose: But genes aren’t the whole story and genetic studies have big limitations.

Leffer: That’s right – especially when it comes to analyzing nuanced human identities like… for instance… sexual orientation. What can genetic research really tell us about the origins of our own attractions, behaviors, and desires?

Bose: Am I genetically predisposed to dying my hair different colors all the time like Clementine in Eternal Sunshine of the Spotless Mind?

Leffer: I’m not sure that scientists have investigated that one, but some researchers have started using genetic study techniques in interesting ways. We’re talking bisexuality, risk-taking behavior, and evolutionary hunches extrapolated a bit too far. I’m Lauren Leffer, contributing writer at Scientific American.

Bose: And I’m Tulika Bose, senior multimedia editor.

Leffer: And you’re listening to Scientific American’s Science Quickly podcast.

Leffer: So Tulika, there’s this recent study that we’ve been chatting about for more than a month now.

Bose: Mhmm, the “risky bisexuals” paper. Or at least, that’s how we've been referring to it.

Leffer: Yeah, and it’s interesting research in some ways. But there were lots of overly simplistic headlines that came out about it. And the work also kind of exemplifies these flaws and limitations that are inherent to a certain type of genetic study.

Bose: Oh man, these headlines: “Groundbreaking new study suggests bisexuals are helping to keep gay people from going extinct”, “Straight Men With ‘Bisexual Genes Have More Kids, Study Finds”, “A New Study Says Bi People Take More Risks We Asked Bi People if It’s True”, “Controversial New Research Find That Bisexauls Are a Bunch of Rascals”... Ok, some of these are pretty funny.

Leffer: Yeah, lots of outlets got kind of cheeky with it. Which fair, I love a fun science story. But here at Scientific American, we still have to make sure that the science is clear, credible, and accurately explained. I spoke with some experts about the research and lots of them had concerns and questions about how the study authors frame their findings and about how work like this can be misinterpreted.

Bose: Ok, Ok– right on. So you’re saying bisexuals might not be genetically-predestined mavericks?

Leffer: Yeah, not quite– but to understand why we’ve got to get into some nitty gritty science and talk about what the study did find. So buckle up because it’s a little wonky.

Bose: [Breathes in] Alright, I’m ready for the explanation.

Leffer: Ok, the two scientists behind this research use a technique called genome wide association study, or GWAS, that compares entire genome sequences from lots of people in a quest to find areas of overlap between genes and certain traits. In the case of this study, they used a subset of participants from a database called the UK Biobank that started up in 2006. Specifically, they looked at the genes and self-reported information from about 450,000 British people– all white and all between the ages of 40 and 69 years old at the time that they participated in the Biobank survey.

Bose: Got it, so we’re only talking about older, white, Brits here.

Leffer: Right, the study only really applies to this very specific group of people. And by looking at the genomes from these hundreds of thousands of participants and cross-referencing their answers to certain survey questions, the researchers found some link between genes and bisexual behavior in men (and only men). They also found that the specific genes correlated with bisexual behavior in men were distinct from the ones correlated with exclusively same-sex behavior.

Bose: So 'gay genes' are separate from 'bi genes'?

Lauren: Sort of. In the simplest terms and according to this one analysis. And remember these are just correlations and we’re talking about self-reported behavior, not peoples’ actual identities or sexual orientations. And sidebar: As a bisexual person, honestly at first I was kind of excited about this study finding. So much of the societal response to bisexuality comes in the form of dismissal or invalidation. People have a lot of trouble accepting that bisexuals exist.

Bose: Uh huh, we are less than 20 years removed from that infamous “Bisexuals-Straight, Gay, or Lying” headline.

Lauren: And initially I was sort of hyped to read a science study that at some level offered biological validation.

Bose: As a fellow bisexual person ... that makes sense.

Lauren: But then I remembered that… wait… I don’t need a scientific study to validate my identity. Like, I know I exist. And then, the whole rest of the study findings also brought me back to reality.

Bose: Ah, Understood… what else did they find?

Leffer: So, here’s where things get a little more convoluted. The researchers found that men with the genes associated with bisexual behavior also had genes associated with a self-reported propensity for risk-taking. And, to clarify: The risk-taking characterisation is from a single yes/no survey question, “Would you describe yourself as someone who takes risks?”

Bose: One question? They based a whole trait analysis off of just one subjective-sounding question?

Leffer: Yeah, you’re not alone in being a little worried by that. More on that later. First the rest of the study findings:

Bose: Wait, there’s more?

Leffer: The researchers did a bunch of comparative analyses including men who had “bisexual associated” genes but only reported opposite-sex behavior. They found that, in these ostensibly straight men, the bisexual genes and the risky genes were also associated with a higher number of self-reported sexual partners. The researchers go on to hypothesize that this could be an evolutionary explanation for why the bisexual gene variants and bisexual behavior persists in the population: Because maybe bisexual genes lead straight men to have more sex and thus more children. In the words of the lead study author Jianzhi “George” Zhang, “The basic finding is that bisexual behavior and number of children are genetically positively correlated.”

Bose: Ok, that sounds like kind of a lot of logical leaps from one point to the next.

Leffer: Yeah, the study authors make some big assumptions, and the outside sources I spoke with weren’t on board with many of them. Steven Reilly, an assistant professor of genetics at Yale University, told me he thought the research could have been conducted and interpreted with much greater care. Joanna Wuest, who is an assistant professor of gender and sexuality politics at Mount Holyoke College, said “It’s a lot of storytelling based around not a lot of data.”

Bose: Some solid inter-academic criticism. That seems healthy. What are the big critiques here?

Leffer: We’re not gonna get into all of them… because um this is Science QUICKLY,

Bose: Yeah not Science Slowly —

Leffer: — But a lot of the critiques apply pretty broadly to this entire type of genetic research. There’ve been a bunch of GWAS examining human sexual behavior in recent years. There was a really big example that came out in 2019, which found genetic factors only seem to account for between 8 and 25 percent of sexual orientation.

Bose: Oh right, that study that was kind of the nail in the coffin for the debunked “gay gene” hypothesis. The 2019 study showed that sexual identity is really complicated and likely the product of lots of different gene bits and also environmental, social, and cultural factors.

Leffer: Exactly, but even with that research, there are caveats. The biggest one being that– again– GWAS can reveal associations but not prove directional effects. Agustin Fuentes, an anthropology professor at Princeton University, told me he believes “GWASs show you much less than some people think they do…they tell you nothing about cause and effect.”

And the correlations that GWASs bring up might not actually be the ones that scientists think they’re studying.

Bose: Wait, what do you mean? Oooh is this about the single-survey question thing?

Leffer: Yeah, and beyond. So people are complicated, identity, personality, and behavior are tricky to pin down and define, and genetic researchers are constructing these proxy categories for human traits (like sexuality or riskiness) that are really limited. In Fuentes’ words, being bisexual “does not necessarily mean a person has had sex with someone with a penis and someone with a vagina.” But the recent study and others define the category that way, based on peoples’ own self-reported sexual history.

Bose: That seems fraught, especially when you’re talking about middle-aged-plus people in a country where same-sex acts were a criminal offense into the 1980s. And like, stigma exists. Maybe social conditions have kept people from engaging in same-sex acts even if they wanted to. Maybe people don’t share accurate sexual histories with scientists.

Leffer: Absolutely, plus even the term “sex” is so subjective. People lie and omit things intentionally, but also respondents might not even agree on what the question they’re answering actually means. Sex to one person...[full transcript]

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