It has been a good year for animals — at least for nonhuman ones. The Economist declared 2019 “the year of the vegan.” Demand for meatless burgers at major fast food chains has grown so fast that producers have been unable to keep up. High-end fashion houses like Gucci and Calvin Klein are now completely fur-free. Even the electric car company Tesla has begun offering a vegan version of its vehicles, with synthetic leather seats and trim.

Increased concern for the rights of animals is not only changing what we eat, wear and drive. It has begun to change our laws. In June, Canada passed a new law many are calling the “Free Willy” bill, making it illegal to keep whales and dolphins in captivity. In the same month, the New York state legislature became the first to pass a bill banning cat declawing, a practice that many animal rights advocates say is cruel and unnecessary. Last year, the New York Supreme Court heard a habeas corpus case for a 47-year-old elephant named Happy who is in captivity at the Bronx Zoo. Steven Wise, the lead attorney, argued that Happy is a legal person, entitled to bodily liberty.

In promoting animal rights, advocates have frequently analogized the plight of animals to human rights issues of the past. While animal rights advocates assert that the protection of animals is a natural next step in the expanding circle of human rights, others view animal rights as a sentimental cause for animal lovers. Some resent that many people seem to care more about the suffering of animals than their fellow humans.

Despite the increasing importance of animal rights in our politics and society, however, we still know very little about why some people support animal rights more than others do, or whether there is a connection between support for animal rights and human rights. To explore that, we conducted a study of both individual attitudes and state policies regarding human and animal rights.

Why do some people support animal rights?

To explain the individual variation in respect for animal rights, we drew on survey data collected by the General Social Survey (GSS) in 1993, 1994 and 2008 from roughly 1,500 Americans. The GSS asked questions both about support for animal rights and human rights, as well as a host of questions that capture various traits we believed might be correlated with support for animal rights. These traits include political ideology, wealth, religious beliefs, and gender.

We found that political conservatives and more religious Americans were less likely to support animal rights. Women were much more likely than men to support them. Most interestingly, however, we found that attitudes about LGBT rights, universal health care, welfare for the poor, improving conditions of African Americans, and supporting birthright citizenship for U.S.-born children of undocumented immigrants were strongly associated with views about animal rights. Some of these effects were very large.

Americans who indicated on a 5-point scale that they strongly favored increasing governmental assistance to the sick, for example, were over 80 percent more likely to support animal rights than those who strongly opposed it. These findings hold even after controlling for a variety of potentially confounding factors — including political ideology.

In other words, people who supported an expansive conception of human rights and welfare were also more likely to support animal rights.

States’ animal rights laws are associated with human rights laws

We also wanted to know whether variations in support for human rights might translate into stronger support for animal rights in law and policy across the 50 U.S. states and the District of Columbia. To measure state-level animal welfare laws, we used the Humane Society of the United States’ (HSUS) “Humane State Ranking” scores. To capture the varying levels of protection each U.S. state has afforded to disadvantaged and excluded human populations, we used the “LGBT policy tally” provided by the Movement Advancement Project (MAP) and the Anti-Defamation League’s (ADL) hate crime statute data.

As in our individual analyses, we found very strong evidence for a connection between animal rights and human rights at the state level. States that afforded stronger protections for LGBT rights and enacted more extensive hate-crime statutes tended to have significantly more animal-friendly laws. These results held when controlling for each state’s economic dependency on animal agriculture, state-level political ideology, state per capita wealth, the religiosity of state residents, and race.

The circle of those entitled to basic rights keeps expanding

These findings suggest that the belief that animals have rights may reflect humans’ underlying conceptions of rights for themselves, rather than being simply the concern of animal lovers.

To be sure, the definition of what it means to be human and to which rights human beings are entitled is constantly evolving and remains contested — as reactions to the Trump administration’s new Commission on Unalienable Rights have made clear. While liberals see the commission as a threat to reproductive and LGBT rights, conservatives see it as a means to promote fetal rights and protect religious liberty.

The biggest shifts in human rights laws and treaties over the last 200 years have come less from an expansion in the number of different rights to which humans are entitled than from the expansion in the categories of humans considered worthy of enjoying those rights. The abolition of slavery, decolonization, women’s suffrage, and the civil rights, disability rights, and the LGBTQ rights movements were not efforts to generate entirely new sets of rights. Rather, they sought to secure for previously marginalized and excluded groups the same rights that others were already enjoying. Animals may one day join that circle.

Yon Soo Park is a PhD student in government at Harvard University.

Benjamin Valentino is associate professor of government at Dartmouth College.