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‘IN Synch

November 18, 2022

Written by: Lauren Mellinger and Julia Raver

Every girl knows what it feels like during “that time of the month” – bad cramps, mood swings, you name it. And it seems that other girls can always tell when it’s time. There have been many studies on the correlations of menstrual cycles between best friends, teammates, or even roommates. But do menstrual cycles really sync up? 

 

[ sea-nsory details ] 

One study claims that menstrual cycles sync based on olfactory cues. So what are olfactory cues anyways? Olfaction, the sense of smell based on chemical cues in the environment, has significant effects on animal behavior (Brady et al., 2012). Specific olfactory receptors within the body react either positively or negatively to different chemical scents. This transfers specific information to olfactory centers (Ruta, 2021). 

These olfactory centers have pathways to the amygdala (which triggers a fight or flight response), lateral hypothalamus (the hunger center which controls feeding behaviors), and the insular cortex (links sensory and emotional experiences).The location where the centers deliver information determines feelings and sensations. For example, it determines when you’re hungry, if you’re swimming, or if you’re happy or sad (Russell et al., 1980). 

A study treated two groups of women on their upper lips with an alcohol-based mixture 3 times a week during a span of 4 months. One group was treated with plain alcohol and the other was treated with a mixture of alcohol and underarm perspiration from a single donor. They found that the women’s cycles treated with the donor’s perspiration scent ended up having significant menstrual cycle shifts towards the cycle of the donor (Russell et al., 1980). 

So one potential source of synching could be exposure to specific chemicals in the environment. Imagine you are constantly working out with your sports team and sweating together, it is possible that your cycles could sync due to the exposure of each other’s underarm perspiration. 

 

[ reely great roommates ]

 

Another study performed by Martha McClintock studied the cycles of undergrad roommates over the entire year. In October, the cycles were on average 8.5 days apart, but, by March, they were about 5 days apart. On the other hand, randomly matched pairs showed no synching (Strogatz, 2022). There was no science, however, behind this discovery, only physical closeness to cite. 

 

[ knot just a phase ] 

A final theory is that lunar cycles play a factor in menstrual synching. The study found that cycles lasting longer than 27 days historically had intermittent synching with both the Moon’s luminance and gravimetric cycles (Helfrich-Förster, 2021). The luminance cycle is the light intensity of the Moon in each lunar phase. The gravimetric cycle is the difference of the Moon’s pull when at different positions with respect to the Earth (Cohut, 2021). 

 

The study found that women with cycles around 29.5 days, or the period of the moon, have the highest likelihood of becoming pregnant. There was also a correlation found between the three lunar cycles – the synodic, the anomalistic, and the tropical month – and the onset of menstrual cycles. These trends however hold much stronger in ancient times. Back then there weren’t as many societal impacts like stress and nutrition on the duration and frequency of the menstrual cycle (Cohut, 2021). So studies point to a historical linkage, although it doesn’t necessarily hold true in today’s time. 

 

[ hook, line, and sink-her ]

Despite all this data and our strong desire to connect over shared pain, estrous synching is a weak phenomenon at best. Researchers have tried over and over — with roommates, best friends, and even rats — and nearly all of them come back with mixed findings. 

[ un-sync-able studies ] 

In the largest study ever taken on, data was collected from 186 women who lived in dorms together for over a year. None of the menstrual clusterings were stable or occurred more than by expected by chance (Yang & Schank, 2006).

A similar study with sorority sisters excluded those taking oral contraceptive and those with irregular cycles. Still no signs of menstrual synchrony (Wilson et al., 2003). 

Among a women’s basketball team, spending time together was actually found to be negatively related to synchrony, as was exercising. If the theory that pheromones are the source of synching cycles was true, exercising with one another should be positively associated with synchrony (Weller & Weller, 2000). 

Data from Clue, a popular period tracking app didn’t support McClintock’s conclusion either. Data from over 1500 users demonstrated that being in close proximity with another menstruator likely does not disrupt your cycle. Over 75% of pairs actually had a bigger difference in cycle start dates by the end of the study (Ray, 2022). Based on this data, your cycle would be more likely to diverge than converge with another person’s!

 

[ like rats abandoning a sinking ship ]

Observing 10 pairs of female rats, unsurprisingly, there was no effect of synchrony (Schank, 2001). Thus, the belief that there is some biological or evolutionary explanation loses its stability, too.

 

[ the synching ship of menstrual synchrony ]

From all this evidence, we can see that the popular belief of “living together, bleeding together” has pretty much been debunked.

Let’s say a typical cycle length is 28 days. The maximum that two women could be out of phase is 14 days. On average, these onsets will be 7 days apart, and half the time they should be even closer. Since the average menstrual cycle lasts 5 days, it’s not surprising that friends or roommates commonly recall experiencing overlaps, falsely confirming the myth (Strassmann, 1999). When we do see overlap, statistics says its due to coincidence supported by math rather than biology.

When you think about how much variation there is not only between menstruators but within each individual, finding proof of synchrony is virtually impossible! There are just too many obstacles in the way that reduce the potential for consistency, like birth control, energy levels, stress, weight, genetics, and age (Strassmann, 1999).

Harsh as it may sound, the studies where synchrony was found may have been plagued with methodological errors (Rudis, 2008). This should not be cause for worry though. Variability between women may be necessary and inherent to our physiology and the process of ovarian cycles (Schank, 2000). In an evolutionary context, avoiding synchrony would enhance female choice among males and reduce competition among females. Within-woman variability would also reduce the chance of males predicting the next ovulation and controlling the matings of females (Schank, 2000).

The claim that cycles dominate on phases of the moon has no leg to stand on either. No one is quite sure if they are set to the full moon, new moon, or somewhere in between.

[ to sync or swim ]

What it comes down to is a simple matter of math and time. I advise you to be careful not to mistake the ebb and flow of cycles over time as synchronization. However…

There’s no harm in bonding over the magical (and miserable) mystery that is menstruation. I think it’s a positive way to find comfort with other women who might be going through the same thing. So, what do you think?

 

 

[ sources ]

https://www.medicalnewstoday.com/articles/menstrual-cycles-and-lunar-cycles-is-there-a-link#:~:text=The%20luminance%20cycle%20refers%20to,it%20orbits%20around%20our%20planet. 

https://modern-cynic.org/2021/01/12/corporate-rats-desert-a-sinking-ship/ 

https://doi.org/10.1080/23293691.2016.1150132

https://doi.org/10.1126/sciadv.abe1358 

https://www.gynecajournalcu.org/blog-articles/staff-writing/do-periods-actually-sync.html

https://www.empowher.com/media/reference/true-or-false-women-who-live-together-tend-have-synchronized-menstrual-periods

https://doi.org/10.1016/0091-3057(80)90020-9 

https://www.rockefeller.edu/news/30838-study-reveals-smell-receptors-work 

https://www.sciencedirect.com/science/article/pii/S0306453000000299

https://www.sciencedirect.com/science/article/pii/S0031938400003954

ttps://academic.oup.com/humrep/article/14/3/579/632869?login=true

https://ieeexplore.ieee.org/abstract/document/638513 

https://modernfertility.com/blog/cycles/

https://www.sciencedirect.com/science/article/pii/030645309500007B

https://doi.org/10.1016/S0306-4530(01)00102-0

https://www.sciencedirect.com/science/article/pii/030645309190021K

https://doi.org/10.1007/s12110-006-1005-z

https://doi.org/10.1007/s12110-006-1004-0 

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About Author

Hi I'm Lauren! I'm a second year Chemical Engineering major with a passion for sustainability. I have always loved reading about current events in science and I can't wait to educate more people about new discoveries in STEM!

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.