Earlier this month voters in two U.S. States, Colorado and North Dakota, considered new laws that would bolster the legal rights of a fetus before birth. Neither of these ballot initiatives passed, but they’re part of a “personhood movement” that’s been gaining notoriety among pro-life advocates since about 2008. Reading about this movement in the press (Vox has a great overview) has made me wonder about the slippery, contentious, and profound meaning of “personhood.”

The Wikipedia page for personhood gives this definition: “Personhood is the status of being a person.” Right-o.

The page for person isn’t much clearer: “A person is a being, such as a human, that has certain capacities or attributes constituting personhood, which in turn is defined differently by different authors in different disciplines, and by different cultures in different times and places.”

I’ve chosen five personhood perspectives to write about this week. Today’s installment is all about conception (another fuzzy concept). Tomorrow I’ll try to tackle the transition from child to adult. Wednesday I’ll ask whether dead bodies are people. Thursday goes to non-human animals, and Friday to neuroscientists who argue that “personhood” is a convenient, if illusory construction of the human brain.

I’d love to hear about how you guys define personhood, and why. Feel free to leave comments on these posts, or jump in to the #whatisaperson conversation on Twitter.

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I went to a Catholic high school, where I was taught in religion class that life begins at conception. I don’t remember my teacher getting into the biological details, but we all knew what she meant: Life begins at the moment that an earnest sperm finishes his treacherous swimming odyssey and hits that big, beautiful egg.

That’s what many Christians believe, and it’s also the fundamental idea behind the personhood movement. The website of Personhood USA, a nonprofit Christian ministry, highlights this quote by French geneticist Jérôme Lejeune: “After fertilization has taken place a new human being has come into being. It is no longer a matter of taste or opinion…it is plain experimental evidence. Each individual has a very neat beginning, at conception.”

That’s not a common belief among biologists, however. Scott Gilbert of Swarthmore calls the conception story a “founding myth,” like The Aeneid. As he jokes in a popular lecture, “We are not the progeny of some wimpy sperm — we are the progeny of heroes!”

In reality, conception — or more precisely, fertilization — is not a moment. It’s a process.

After the sperm DNA enters the egg, it takes at least 12 hours for it to find its way to the egg’s DNA. The sperm and egg chromosomes condense in a coordinated dance, with the help of lots of proteins call microtubules, eventually forming a zygote. But a true diploid nucleus — that is, one that contains a full set of chromosomes from each parent — does not exist until the zygote has split into two cells, about two days after the sperm first arrive.

So is that two-cell stage, then, at day two, when personhood begins?

It could be, if you define personhood on a purely genetic level. I have a hard time doing so, though, because of twins. Identical twins share exactly the same genome, but are obviously not the same person.

Based on this logic, some biologists push back the start of personhood to about 14 days after the sperm enters the egg, a stage called gastrulation. This is when the zygote transforms from one layer into three, with each layer destined to become different types of tissues. It’s only after this stage that you could look at a zygote and say definitively that it’s not going to split into identical twins (or triplets or even quadruplets).

View Images Via Wikipedia: Gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. Image via Wikipedia

So is the 14th day of gestation, then, when personhood begins?

Some doctors would say no, you have to also consider the fetal brain. We define a person’s death, after all, as the loss of brain activity. So why wouldn’t we also define a person’s emergence based on brain activity? If you take this view, Gilbert notes, then you’ll push personhood to about the 28th week of gestation. That’s the earliest point when researchers (like this group) have been able to pick up tell-tell brain activity patterns in a developing fetus.

Most legal definitions of personhood in the United States also focus on this late stage of gestation. The famous Roe v. Wade case in 1973 made it illegal for states to ban abortions before the third trimester of pregnancy, which begins at 28 weeks. Subsequent rulings by the court got rid of this trimester notion, saying instead that abortions can’t happen after a fetus is “viable,” or able to live outside the womb, which can be as early as 22 or 23 weeks. (And in 2003, Congress banned a specific procedure called a partial-birth abortion, which happens between 15 and 26 weeks.)

So there you have it. From a biological perspective, neither conception nor personhood is easily defined. “I really can’t tell you when personhood begins,” Gilbert says in his lecture. “But I can say with absolute certainty that there’s no consensus among scientists.”