How Chemicals Affect the Brain & Sexual Preference | Dr. Shanna Swan & Dr. Andrew Huberman

00:00:00.000 | Maybe we could take a moment and talk about atrazine and its effect on male sexual behavior

00:00:08.000 | in amphibia to the sperm studies because Tyrone Hayes is a wonderful researcher who established

00:00:16.680 | a link through his research between atrazine exposure and male sexual behavior of amphibia.

00:00:25.160 | Could you elaborate on that?

00:00:26.760 | Tyrone first caught frogs in the wild, in environments that were more or less exposed

00:00:33.360 | to atrazine, and showed effects on development and sexual behavior.

00:00:38.640 | Then in his lab, he actually exposed them.

00:00:41.680 | So he knew exactly who was exposed and how much.

00:00:45.360 | And he showed that, and I can't tell you what percent or what, you know, but a significant

00:00:51.840 | number of frogs exposed to this pesticide atrazine chose to mate with other male frogs.

00:01:00.200 | Tried to mate with other male frogs, presumably unsuccessfully.

00:01:02.680 | Well, they mounted them.

00:01:04.800 | He has photos of the males mounting males.

00:01:08.360 | That's a remarkable result.

00:01:10.120 | It's been kind of, you know, used and misused out there in the media and in popular culture.

00:01:18.380 | But if nothing else, it suggests that the organization of the neural circuits and neuroendocrine

00:01:23.240 | pathways that control sexual, I don't want to say partner, because it's mating, frogs

00:01:31.360 | aren't monogamous, but sexual preference are significantly impacted by this atrazine.

00:01:40.360 | Yes.

00:01:41.360 | And it suggests that there are other environmental chemicals as well.

00:01:46.560 | And I don't know if we'll have time to go there, but I did work on neurodevelopmental

00:01:54.100 | outcomes in relation to prenatal phallic exposure.

00:01:57.000 | And so I think the overarching idea here is that the brain, like the genitals, is sexually

00:02:05.360 | dimorphic.

00:02:07.200 | And there's many people, by the way, who will take offense at that.

00:02:10.600 | Really?

00:02:11.600 | Yeah.

00:02:12.600 | I think there's, I mean, going back to the work of Frank Beach in the psychology department

00:02:16.240 | at UC Berkeley, showed this in beagles.

00:02:19.680 | It's been shown in pretty much every species.

00:02:22.800 | But it's not a better or worse.

00:02:23.920 | I think this is what people need to hear, like dimorphic does not mean better or worse.

00:02:26.760 | It means different.

00:02:27.760 | Right.

00:02:28.760 | Right.

00:02:29.760 | And that there are, for example, advantages to spatial reasoning in a male, which are

00:02:39.920 | related to testosterone, right?

00:02:42.120 | You know that.

00:02:43.120 | I mean, I think there, yeah, I mean, my understanding of this literature, and I'm not an expert

00:02:46.640 | in this particular aspect, which is the behavioral phenotypes, but, you know, like the medial

00:02:52.880 | preoptic area of the hypothalamus is known to be sexually dimorphic, dependent on testosterone,

00:02:57.480 | converted into estrogen during development, et cetera, et cetera.

00:03:00.280 | And there's just so much evidence of this.

00:03:04.460 | How it links to behaviors, I think, can be reasonably placed into ethologically relevant,

00:03:13.060 | evolutionarily logical arguments when talking about rodents or beagles or even rhesus macaque

00:03:19.980 | monkeys.

00:03:21.660 | I think where people get a bit inflamed is when people try and take the sexual dimorphisms

00:03:28.680 | that have been observed in animal brains, or even in human brains, and tack those to

00:03:33.440 | specific abilities or lesser abilities.

00:03:37.420 | I think that's when people sort of go, wait a second, like, I have much better sense of

00:03:40.800 | direction than my husband.

00:03:41.800 | And you go, well, yeah, like, you know, and then you go, well, does that mean that she

00:03:45.400 | has higher testosterone than him?

00:03:46.760 | And then maybe, and then, and pretty soon you're in almost a no man's land, a no person's

00:03:53.360 | land of confounding variables, right?

00:03:57.840 | And I really appreciate that you raised this.

00:03:59.800 | And also that you said it and I didn't, because I feel safer that way.

00:04:02.840 | But look, there is a very simple, outdated questionnaire, and it's play behavior.

00:04:11.260 | It's called the PSAI.

00:04:12.600 | It's been used for years.

00:04:13.600 | Have you heard of it?

00:04:14.600 | It's a rough and tumble play.

00:04:16.640 | Yes.

00:04:17.640 | Yeah.

00:04:18.640 | Yes.

00:04:19.640 | And there are 24 questions on there, and they are sexually dimorphic, I guess you could

00:04:24.460 | say that, you know, my child likes to play with dolls, my child likes to play dress up,

00:04:29.660 | my child likes to play rough and tumble, et cetera.

00:04:33.920 | And we gave that questionnaire to our population and looked at the answers that the mothers

00:04:43.400 | gave, both in our population, by the way, and a Swedish population of a colleague there,

00:04:48.400 | Carl Bornahag and Gustav Bornahag.

00:04:53.520 | And what we found, higher phthalate levels, these anti-androgenic phthalates were associated

00:05:03.040 | with less masculine male typical play in our male boys.

00:05:10.120 | So-

00:05:11.120 | This is phthalate exposure to the mom, baby is born in the young human child.

00:05:18.320 | Yeah.

00:05:19.320 | I think it was four years of age.

00:05:20.320 | Four years of age.

00:05:21.720 | Less rough and tumble type play among the boys whose mothers were exposed to more phthalates

00:05:27.320 | during a critical period of development.

00:05:29.940 | Now you can see that's a politically loaded issue now.

00:05:34.040 | I mean, you know, well, I think we're, I mean, let's have some fun with this in the scientific

00:05:41.240 | sense.

00:05:43.360 | The notion of dimorphism is, you know, okay, male and female brains are different, right?

00:05:47.640 | And male, female defined in those, almost all those studies as presence of a Y chromosome.

00:05:52.200 | And then people say, well, there's some, there's XYY, and then there's XXY, okay.

00:05:55.600 | But most of the time you're talking about XX chromosome or XY chromosomes at birth.

00:06:00.440 | Forget everything else for the moment.

00:06:04.340 | These are always distributions.

00:06:06.020 | This is what I think people need to know.

00:06:07.440 | We're not talking about, these are not, this is not, you know, two hills of data separated

00:06:13.100 | by a valley.

00:06:14.440 | These are overlapping distributions, right?

00:06:16.960 | So you get males with a "female-like" distribution, you get females with a "male-like" distribution.

00:06:22.960 | And I think as long as we acknowledge that, then we're just talking statistics.

00:06:28.760 | We're not placing any cultural or any value on it really whatsoever.

00:06:35.480 | But if you could make the analog to the intergenital distance, it's kind of similar.

00:06:39.720 | You know, you have the same exposure, phthalate exposure, you have something changed statistically.

00:06:43.960 | We don't see huge differences in the boys' genitals.

00:06:49.440 | And we don't see huge, I don't, we know, these kids have not been scanned, so we don't know

00:06:53.320 | how their brains look.

00:06:54.320 | But based on their answers, we don't see huge differences.

00:06:57.940 | We see tendencies.

00:06:58.940 | We see they are more likely, if they had been exposed to these phthalates, to want to play

00:07:05.160 | dress up and have tea parties.

00:07:07.840 | More likely.

00:07:08.840 | Doesn't mean that they're all going to, but that's the direction and so on.

00:07:13.280 | So I think we have to just think about more likely, not absolute.

00:07:16.480 | [music]

How Chemicals Affect the Brain & Sexual Preference | Dr. Shanna Swan & Dr. Andrew Huberman

Chapters