Like owner, like dog. Pets may share more than their owners’ overall personality. Historical hints at the stunning physical resemblance of people and their animals may have been scientifically founded after all: the same artificial substances affecting our metabolism may be triggering the same reaction in dogs and cats as well.

Endocrine-disrupting chemicals may alter the balance and mechanisms our metabolism uses to fight the need (or excess) of nutrients, hence their name, “obesogens,” proposed by a biology professor at the University of California, Irvine, Bruce Blumberg.

Not only diet, behavior, and heredity may be responsible for weight gain across the board during the last decades. By destroying the cells’ ability to turn lipids into energy, such substances could be speeding up several of the most pervasive health issues.

Scientists studying the impact of endocrine disruptors in obesity are pointing at pets to test their hypothesis: why is it that it affects cats, dogs, and other animals sharing our environment? Despite the difficulty of “determining what fraction of obesity is related to chemical exposure,” as Bruce Blumberg points out, some facts reinforce the hypothesis: laboratory animals, from rodents to primates, are also experiencing a rise in obesity despite being raised “under strictly controlled conditions of caloric intake and exercise,” writes Mark Buchanan, science writer at Bloomberg:

“The only possible factors driving weight gain for these animals, researchers believe, would be subtle chemical changes in the nature of the foods they eat, or in the materials used to build their pens.”

The way our pets look

Those doubting the impact of nurture over nature in development will have a hard time acknowledging the astonishing look-alike of pets and their owners: in their development, pets share the environmental contingencies that affect the biology and psychology of people, so why would pets be isolated from the impact of coexistence with others?

Or, to the clinical eye of the British writer and physician Arthur Conan Doyle:

“A dog reflects the family life. Whoever saw a frisky dog in a gloomy family or a sad dog in a happy one? Snarling people have snarling dogs, dangerous people have dangerous ones.”

Recent studies hint and the ancient relationship between canids and stone age hunter-gatherers: dogs’ domestication seems related to the mutual benefit of both species when hunting together in challenging Ice Age environments. If not refuted, the hypothesis of our shared survival strategy could explain their loyalty and uncompromised devotion towards their owners.

Raining cats and dogs

Science fiction writer and counterculture pioneer Aldous Huxley put it another way that feels more contemporary and that explains dogs’ popularity even among those uninterested in hunting or exploring the outdoors:

“To his dog, every man is Napoleon; hence the constant popularity of dogs.”

Of course, humans of all ages have also noticed how cats, our other ancient companions, behave differently than dogs. Cats’ domestication seems linked to the arousal of the first permanent neolithic settlements in the Fertile Crescent.

Population density may have caused an explosion of the house mouse and the threat it caused to human health and grain storage; hence the opportunity of “tolerating” small felines’ legendary autonomy, which would remain unchallenged, in exchange for the most strategic service: fighting animal pests in the first cities.

British-American journalist and professional provocateur Christopher Hitchens did indeed comment about the dog-cat divide and what it hints about the choice of one over the other:

“Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods.”

This could lead us to ironically speculate that cats may feel their service is still required, and their human counterparts should tolerate such aloofness. In contrast, dogs may bear the curse of sensing that their own “call of the wild” is not necessary anymore as hunting or sledging in Arctic areas become luxury activities only a few (and their dogs) can perform.

The surroundings (and leaky products) we share

One picture made the memes amid Elizabeth II’s death: in it, we can see her corgis dogs being carried out of a plane returning from Scotland after their owner’s death. Several generations of corgis dogs, a fearless little-sized breed that evolved herding cattle, surrounded Elizabeth since she was seven years old, mimicking their owner’s unassuming sobriety in pictures. If pets seem to share their owners’ personality aura, they often show similar signs of rising contemporary diseases such as overweight, depression, or behavioral disorders.

Psychologists have explored the notion of why humans seem to resemble their pets, especially their-markedly more empathic-dogs. Sadahiko Nakajima, a researcher from Japan’s Kwansei Gakuin University, conducted a 2009 experiment in which people could match dogs and their owners at a rate consistently higher than chance. His experiment gathered 500 participants who showed the difficulty of matching animals with people when the eyes of humans or dogs were blacked out.

Michael Roy, a University of California, San Diego psychologist, photographed dogs and owners separately and asked study participants to match them. Participants could associate people and their pets with reasonable accuracy, and similar experiments with identical outcomes have been replicated several times.

Roy believes this result is related at least in part to the human narcissistic need to look for people, pets, or objects with an allure of familiarity we can relate to. In the same way, we feel attracted by people we instinctively feel compatible with us, both physically and personality-wise.

Yet there’s more than the apparent behavioral mimicry arising from a pet’s education and its coexistence within a household—including the level of activity and diet—; something that also goes beyond the unconscious preselection a person may perform when adopting a puddle: some products that have infiltrated our households and environment over the decades may be affecting us—and our pets—in a similar way.

A moment of (plasticky) solace

The pervasive use of plastic polymers and substances associated with its mass production since the rise of petrochemicals after World War II may have played a role in health issues in humans and their closest animals, surrounded by products of everyday use, including substances that are known to interfere with key mechanisms of the endocrine system in vertebrates.

As William McDonough and Michael Braungart explain in the memorable opening of Cradle to Cradle: Remaking the Way We Make Things, their 2002 book on how to build sustainable and more healthy ways for the houses and products that surround us, the safe and comfortable place we have created at home or work may not be as convenient after analyzing the chemical composition of the surfaces and objects we interact with.

“At last. You have finally found the time to sink into your favorite armchair, relax, and pick up a book. Your daughter uses a computer in the next room while the baby crawls on the carpet and plays with a pile of colorful plastic toys. It certainly feels, at this moment, as if all is well. Could there be a more compelling picture of peace, comfort, and safety?”

The second paragraph announces trouble: “Let’s take a closer look.” Braungart and McDonough dissect the elements (conventional furniture and objects, made with common, absolutely legal substances and materials) in the scene —and their potential effect: a comfortable chair that contains mutagenic materials, heavy metals, chemicals, and dyes; a computer containing toxic gases, cadmium, lead, mercury, acids, plastics, chlorinated and brominated substances; printer toner cartridges containing nickel, cobalt, and mercury; a downcycled carpet made of leaky plastic polymers; shoes fabricated overseas where health standards are much lower; a PVC plastic baby toy (potentially, with phthalates, dyes, lubricants, antioxidants, and ultraviolet-light stabilizers.

“So much for trying to maintain a healthy environment or even a healthy home. So much for peace, comfort, and safety. Something seems to be terribly wrong with this picture.”

Killing the messenger

If the opening of Cradle to Cradle sounds fear-mongering to some, the background of one of its authors, German chemist Michael Braungart, suggests the thread is nothing but serious. Kurunthachalam Kannan, professor of environmental sciences at the University at Albany, State University of New York, documented in one 2010 study several hazardous organotins in a designer handbag, wallpaper, vinyl blinds, tile, and vacuum cleaner dust collected from 24 houses.

It’s hard to avoid the impact in our lives of flame retardants (found in carpeting and furniture), bisphenol A (BPA, a soluble hormonal disruptor used in pasta, fruit, and vegetable canned fruits, as well as several plastics), phthalates (plasticizing agents used in paints, cosmetics, and medicine), parabens (widely used in food preservatives, paper products, and cosmetics), organotins (antifungal agents found in plastic products and clothes), or pesticides such as glyphosate.

As biology professor from the University of Missouri who has studied the effects of BPA over three decades, Frederick vom Saal has seen how BPA reduces the number of fat cells but the ones remaining incorporate more fat. Fewer, very large cells may affect health in many ways:

“In animals, BPA exposure is producing the kind of outcomes that we see in humans born light at birth: an increase in abdominal fat and glucose intolerance.”

Intergenerational disruption

By potentially disrupting the hormonal balance in us, our pets, and wildlife, such substances present in the urine of most adults also impact our mood and weight. Chemical toxicity can also affect several generations, as chemical exposition appears in epigenetic mechanisms that may impact future generations: experiments in mice show how tributyltin (TBT), present in wood preservatives, impacted hormonal reactions in offspring:

“If you give tributyltin [TBT] to pregnant mice, their offspring are heavier than those not exposed,” Blumberg says. “We’ve altered the physiology of these offspring, so even if they eat normal food, they get slightly fatter.”

As substances such as TBT enter our bloodstream, they alter the biochemistry of our endocrine system, which regulates all the biological processes during life. The efficiency of this “messenger” system depends on its ability to interpret chemical states in our body since it will order the release of hormones depending on the apparent need of distant organs. As their name illustrates, endocrine disruptor substances alter this precise chemical ability within vertebrates.

Studies have documented the presence of TBT in human blood, milk, and liver samples. This isn’t good news to Blumberg, who was studying endocrine disruptors in the early 2000s when somebody pointed at his studies linking TBT to sex reversal in multiple fish species. And, when exposed in utero to TBT, even in that organism is never exposed again, the effect caused is permanent.

Learning how to read labels

By overriding our endocrine system, we risk damaging vital one vital function our health depends on: the production and release of hormones in charge of mood, development, and growth, among others. And, if our endocrine system loses its wait, so will our behavioral impulsivity —and our ability to know whether we need to keep eating or not since our altered appetites won’t be able to tell satiety from hunger anymore.

Once dysfunctional, our messenger system will have a hard time guessing what our gastrointestinal tract, pancreas, and liver need. Obesogens are the environmental link to obesity that individuals and public health have not taken seriously yet. As obesity rises steadily in the human population, as well as in pets, urban rats, and laboratory animals, scientists studying the phenomenon warn that the generalized trend doesn’t depend only on changes in diet and exercise. As explained by Robert H. Lustic, a clinical pediatrician at the University of California, San Francisco:

“Even those at the lower end of the BMI [body mass index] curve are gaining weight. Whatever is happening is happening to everyone, suggesting an environmental trigger.”

Acknowledging that dietary, pharmaceutical, and industrial compounds alter metabolic processes and predispose people to gain weight explains only a part, though fundamental, of the steady rise of obesity around the world.

The Environmental Working Group (EWG), a watchdog group specializing in toxic chemicals in food and other consumer products, publishes a guide to endocrine disruptors.