**1. Introduction**

There are about 1.5 billion vegetarians worldwide, only 75 millions of whom are so by choice. The others are vegetarians out of necessity. Most of the latter will start eating meat when they can afford it [1]. In this chapter we will focus on people who are vegetarian/vegan of their own free will.

The earliest description of the vegetarian diet dates back to 800 BC and is related to Buddhism and Hinduism in ancient India, whose basic belief is in the unity of all living beings. *'Ahimsa'* or non-harming is a strong element in these religions. Today, people who favour ancient Indian lifestyles or habits such as yoga, are also in favour of plant-based diets. For example, 30% of UK yoga teachers follow a plant-based diet, which is 25 times the proportion in the general UK population [2].

In the western world, Pythagoras, the ancient Greek philosopher and mathematician formulated his ideas about reincarnation, which, as a consequence, led him to avoid eating meat. At the same time, around 600 BC a number of other religious groups (e.g. Orphism) also banned eating animal products.

Pythagoras can be considered the father of ethical vegetarianism and this is where the term *Pythagorean way of life* originates. Pythagoras had followers among influential philosophers and writers and his influence on nutrition in Europe continued until the 19th century.

In medieval Europe there had been no real popular interest in vegetarian food except among some philosophers and writers. The first Vegetarian Society in England was only founded 1847, with other countries following [3]. Interest in plant-based diets have been growing ever since.

Today, people choose plant-based diets from different motives, such as health, taste/disgust, animal welfare, environmental concerns, and weight loss [4]. One reason for choosing a plant-based diet is a healthy lifestyle [5], plant-based diets give some protection against the risks of developing somatic diseases [6]. For example, a vegetarian diet has a protective effect on the incidence of and mortality from heart diseases and cancer [7] and the risk of diabetes [8, 9].

The choice of a vegetarian diet is often related to how people see themselves and how they see others. Their diet has become part of their identity [10]. Nowadays, a growing number of people eat a plant-based diet because they are concerned about climate change [3].

It is not surprising, therefore, that psychological research on everyday eating habits and changing behaviours, lifestyle aspects and their consequences is growing [11]. A recent study found a possible relationship between perceived masculinity and diet preferences with some (weak) evidence that veganism (slightly stronger for males) leads to perceptions of decreasing masculinity when compared to omnivores [12].

People have different conceptions about food and lifestyle. Because of differences in personal and psychological characteristics there may be a difference between people who use different diets. This chapter discusses whether eating a plant-based diet influences mental state, and if so: is it different from that of people with other eating habits?
