In Zoopolis, Sue Donaldson and Will Kymlicka discuss a political philosophical theory of animal rights. According to this theory, all sentient beings, including animals, can be divided into three categories that we currently apply to humans.
The first category consists of the citizens who should get all the meaningful civil rights (in this sense, mentally disabled humans count as citizens). The citizens in Zoopolis are basically all humans and domesticated animals, or more precisely: everyone whose birth (or existence) is largely our responsibility. (By “our” I mean all beings who capable of making reasoned choices and thus bear a moral responsibility. In other words: all rational beings). Citizens are thus those sentient beings whose birth is determined by our choices. With citizens, we can know who is born, and how many are born, because a birth registration of all citizens is feasible. For all those beings we have a particular responsibility, because we are responsible for their very existence.
The other two categories consist of beings whose existence we have not determined. Birds, hedgehogs, mice and other animals are nearby residents who reside in our neighborhoods, but these beings cannot be registered in a birth registration, because we do not know who is born where and when. We can compare this category with foreign tourists, immigrants and asylum seekers residing in our country. Those people do not have the full rights that the first category has, but they still have a number of highly important basic rights. The third category consists of wild animals in nature. The wild parks can be considered as sovereign states, having the rights of sovereign states.
With this background in political animal philosophy, we can do a thought experiment: the birth lottery. This is a form of the “veil of ignorance” in the theory of justice of the political philosopher John Rawls. Suppose you will soon be born (or reincarnated), but at this moment you do not know who you will be, as if you are behind a veil of ignorance. You may now, behind, choose which moral and social rules everyone should comply with. What rules would you want? Because we can only know how many citizens are born in a year, we limit the thought experiment to all citizens, i.e. all humans and domestic and farmed animals. (Ideally, we would have to extend the thought experiment of the birth lottery to all creatures, as discussed in my PhD thesis https://stijnbruers.wordpress.com/2014/04/04/born-free-and-equal-on-the-ethical-consistency-of-animal-equality/, but here I choose to restrict it to citizens for didactic simplicity.)
Imagine a big wheel of fortune (a wheel of births). You may turn the wheel only once, and you will be born as the selected animal. Who will you be? Whose life would you most likely lead? The following graph shows the wheel of births.
The numbers indicate how many of those beings are born each year. With a probability of almost nine out of ten you would be born as a chicken, and then probably a broiler chicken who will have a short life of several weeks, with many diseases, stress, anxiety, pain and growth disorders. Also most of the other citizens do not have a nice life either. With only a 0.24% chance you would be born as a human, and with a probability of 0.17% as a dog or cat (as pet animals). Certainly not all the humans and pets live a happy life in our society. So if behind the veil you would choose the moral and social rules of our current society, you would have a probability less than 0.4% of leading a flourishing happy life.
In other words: there is 99.9% chance that you will be born as someone who has an unpleasant life. With a probability of 98% you will be born as someone who is going to be used for food in a way that you are not going to like. Therefore, behind the veil you would likely choose some other moral and social rules, such as veganism. We should eat a plant based diet.