I recently attended a conference on immigration policy. As is often the case, many of those speakers who favor greater openness to immigration argued that advocates for immigrants should emphasize efforts to allay concerns that immigrants threaten the self-interest of native-born Americans. On this theory, more must be done to explain how immigrants can bolster the economy, spur innovation, and contribute to the culture, and conversely why they don't threaten natives' jobs, increase crime, or overburden the welfare state. These issues are all worth addressing. But in focusing primarily on these sorts of questions, I fear many immigration advocates may be overlooking the lessons of past successful efforts to expand liberty and promote equality for groups previously considered unworthy of serious moral consideration.

Relevant historical parallels include the abolition of slavery, the civil rights movement, the women's rights movement, and—most recently—the successful effort to institute same-sex marriage. In each case, success was achieved principally through arguments focused on universal moral principles and on the common humanity the rest of us share with the group in question, not arguments emphasizing how helping the oppressed would promote the narrow self-interest of other parts of the population. Moral appeals were by no means the only factor that mattered in these cases. But their role was central, nonetheless.

What did more to promote the antislavery cause: the abolitionists' argument that slavery was unjust because black slaves had the same right to liberty as whites, or Hinton Helper's argument that ending slavery would serve the self-interest of southern whites? The answer seems pretty clear. Few if any historians would argue that the latter had more than a fraction of the effect of the former, even though there was considerable truth in Helper's claims. The famous British antislavery image pictured above is another noteworthy example of the key role of moral appeals. It emphasizes that blacks are fundamentally akin to whites, and that there is no justification for denying the former the liberty claimed by the latter. Harriet Beecher Stowe's novel, Uncle Tom's Cabin – the most successful antislavery book in American history, made much the same sort of case. It emphasized the common humanity between blacks and whites and the ways in which slavery was unjust, not the ways in which it damaged the economy or otherwise harmed the interests of whites.

The civil rights movement is a similar case. Martin Luther King's famous argument that segregation should be ended because people should be judged "not be judged by the color of their skin but by the content of their character" was much more effective than economists' (well-grounded) arguments that ending segregation would help jumpstart the southern economy, thereby boosting white incomes.

Much the same story can be told about the women's rights movement, same-sex marriage and other similar examples. Like the civil rights movement, these efforts succeeded largely because a sufficient number people were persuaded that it is wrong to deny people liberty and fundamental human rights on the basis of what are ultimately morally arbitrary characteristics: race, ethnicity, sex, and (most recently) sexual orientation.

Immigration restrictions have important commonalities with past laws restricting freedom based on race, ethnicity, and gender. Like the former, they are largely based on an immutable characteristic stemming from ancestry: in this case, where you were born and whether your parents happen to be US citizens. Both types of laws essentially punish people for choosing the wrong parents.

Like race of birth, place of birth is an immutable characteristic that we cannot change, and one that says nothing about our intrinsic moral worth or how much liberty we should be entitled to. In both cases, discriminatory policies resulted in massive restrictions on the freedom and welfare of the people targeted, all enforced through government coercion. Immigration restrictions, like Jim Crow laws, forcibly confine many of their victims to lifelong poverty and oppression. As economist Alex Tabarrok puts it, "[h]ow can it be moral that through the mere accident of birth some people are imprisoned in countries where their political or geographic institutions prevent them from making a living?"

The parallels between racial discrimination and hostility to immigration were in fact noted by such nineteenth century opponents of slavery as Abraham Lincoln and Frederick Douglass. These similarities suggest that moral appeals similar to those made by the antislavery and civil rights movements can also play a key role in the debate over immigration.

Moral appeals were in fact central to the two issues on which public opinion has been most supportive of immigrants in recent years: DACA and family separation. Overwhelming majorities supporting letting undocumented immigrants who were brought to America as children stay in the US, and oppose the forcible separation of children from their parents at the border. In both cases, public opinion seems driven by considerations of justice and morality, not narrow self-interest (although letting DACA recipients stay would indeed benefit the US economy). Admittedly, these are relatively "easy" cases because both involve harming children for the alleged sins of their parents. But they nonetheless show the potency of moral considerations in the immigration debate. And most other immigration restrictions are only superficially different: instead of punishing children for their parents' illegal border-crossing, they victimize adults and children alike because their parents gave birth to them in the wrong place.

The key role of moral principles in struggles for liberty and equality should not be surprising. Contrary to popular belief, voters' political views on most issues are not determined by narrow self-interest. Public attitudes are instead generally driven by a combination of moral principles and perceived benefits to society as a whole. Immigration is not an exception to that tendency.

This is not to say that voters weigh the interests of all people equally. Throughout history, they have often ignored or downgraded those of groups seen as inferior, or otherwise undeserving of consideration. Slavery and segregation persisted in large part because, as Supreme Court Chief Justice Roger Taney notoriously put it, many whites believed that blacks "had no rights which the white man was bound to respect." Similarly, the subordination of women was not seriously questioned for many centuries, because most people believed that it was a natural part of life, and that men were entitled to rule over the opposite sex. In much the same way, today most people assume that natives are entitled to keep out immigrants either to preserve their culture against supposedly inferior ways or because they analogize a nation to a house or club from which the "owners" can exclude newcomers for almost any reason they want.

Often, such attitudes are not the result of malevolence or stupidity. For many people, they are simply assumed to be part of the natural and inevitable order of things. But of course the same was also true of slavery, racial segregation, and the subordination of women to men. All were also once widely considered an obviously valid part of the natural order, and all were widely supported by generally good and intelligent people. Over time, most westerners came to see that these institutions in fact severely restrict freedom on the basis of morally arbitrary characteristics. As Alex Tabarrok reminds us, "[w]hen in 1787 Thomas Clarkson founded The Society for the Abolition of the Slave Trade… slavery was considered by almost everyone as normal, as it had been considered for thousands of years and across many nations and cultures." Yet, within Clarkson's lifetime, the moral movement he and a few others began led to the abolition of slavery in the British Empire, and eventually helped inspire its abolition in the United States as well.

Effecting a similar transformation in attitudes should be the main long-term goal of immigration advocates. It is not going to be a quick or easy task. But history suggests that it is doable.

History also suggests that it is often easier to get people to expand their moral horizons by building on preexisting intuitions than to consider complex empirical evidence on the effects of government policies on various groups' self-interest. While voters are not, for the most part, selfish, they are "rationally ignorant" about details of government policy. For this reason, among others, it was easier to get whites to see that blacks are fundamentally similar to them than to get them to understand, for example, that letting black workers enter the job market on an equal basis with whites, would ultimately increase white incomes rather than decrease them. The same point applies to moral and empirical arguments about immigration.

None of this indicates that immigration advocates should completely ignore the self-interest of natives. Some issues related to latter are intrinsically important, entirely aside from considerations of political strategy. In addition, people will often ignore or bend moral principles if they believe that is the only way to avoid some horrible disaster. Furthermore a key part of the reason why many fear those of different races or ethnicities is the perception that "we" are enmeshed in a zero-sum game with "them" in which the only way one group can prosper is at the expense of the other. In this crucial sense, moral attitudes cannot be neatly separated from empirical ones. It is easier to expand the scope of the relevant "we" if those added to it are not seen as a dire threat. Even the most effective moral appeals, therefore, are unlikely to end all significant opposition to immigration. But they can go a long way.

They can also make it easier to address fears about the effects of immigration in more humane ways. A person who recognizes that blacks are entitled to the same liberty as whites might still have concerns about large-scale integration. But he or she is likely to be more open to finding ways to address them by means less cruel than barring blacks from white society. If there is a moral presumption against racial discrimination, that means we are obligated to carefully consider other ways of addressing social problems before resorting to it. Similarly, those who accept a moral presumption against migration restrictions based on parentage and place of birth will be more open to considering "keyhole solutions" that address possible negative side-effects of immigration by means less draconian than barring immigrants.

Like slavery, racial segregation, and the subordination of women, immigration restrictions are a complex, multifaceted issue. Advocates for immigrants should not forget that. But they should also keep in mind the crucial role of moral appeals. Like the slave, the migrant fleeing oppression in search of a better life is also "a man and a brother."