A A call for people to eat less meat to save the planet is growing louder, so if less is good, wouldn't none be better? To find out, let's imagine what would happen if the world eliminated meat, milk and eggs from its diet, then trace the effects. Last year, the world consumed 282 million tonnes of meat, 700 million tonnes of milk and 1.2 billion eggs. Environmentally speaking, this came at an enormous cost.

B Agriculture damages the environment. Take, for example, the felling of forests and the widespread use of irrigation systems; it may surprise you to know that agriculture stores more greenhouse gases than all methods of transport put together. Livestock farming does the most damage. In part that is because most livestock eat grain that could be used to feed human populations, and farmers are forced to grow more than we would otherwise need to meet the demand. Altogether, if we switched to a vegan diet, meaning no meat, dairy or eggs, the land currently required for crops would drop by an estimated 21 per cent, about 3.4 million hectares, roughly the size of India.

C One environmental impact that would also lessen through a reduction in animal farming would be nitrogen emitted from agricultural processes, which spread over both waterways and land. According to environmental scientist Alison Leach, if everyone eliminated dairy products and eggs, this kind of pollution would fall by 60 per cent. Livestock production has another serious environmental impact: by 2010, the US accounted for 55 per cent of erosion, mainly from forests being cut down to make way for grazing land. On top of this, half of all antibiotics manufactured are fed to animals as part of their normal diet, a practice creating resistant bacteria.

D A meat-free world, then, would be greener in many ways. However, if everyone opted to give up meat, there would be significant costs too. For most of human history, stock grazed on land that wasn't suitable for ploughing, and in doing so they converted inedible grass into edible meat and milk. Even today, a flock of sheep or goats can be the most efficient way to get food from marginal land. In a world in which over a billion people do not have enough to eat, using this land for crop production would contribute to food insecurity.

E In some parts of the world today, livestock such as chickens can subsist on leftovers and whatever they find. Tara Garnett, who heads the Food Climate Research Network at the University of Surrey, points out the use of the animals. By giving them your leftovers, she says, they deal with your rubbish, and you get meat. Garnett adds, though, that if this kind of approach were eventually adopted, it would require a major adjustment in food preference: people would have to get used to chicken, for example, with less fat.

F Another downside to a meat-free world would be the disappearance of animal by-products. Such a world would have to replace the 11 million tonnes of leather and 2 million tonnes of wool that come annually from livestock farming and which are turned into clothing. Furthermore, even ardent vegetarians acknowledge that dairy products and meat may not be a good thing in poorer countries. "Whilst there's no doubt that considerable reduction of meat consumption would have an environmental benefit, we do have to be careful about saying it would be the best solution if the whole world were vegetarian," says Annette Pinner, chief executive of the UK Vegetarian Society. "For many of the world's poorest residents, animals may present their only realistic hope of an extra income, and a little animal protein can make a big difference to a marginal diet."

G What if we decided on a no-meat but vegetarian diet, rather than a vegan diet? After all, milk and eggs are very efficient ways of producing animal calories. "It's difficult to switch to a no-meat-but-milk diet," says Helmut Haberl, a social ecologist at the University of Vienna. Dairy cows must calve every year to keep producing milk, and only half their offspring will be female. While many vegetarians make it a moral rule not to kill and eat the males, there is surely no practical reason to waste so much meat. Similar arguments apply to chickens kept for eggs.

H So even though a meat-free world sounds good on paper, it is likely that any utopian future will have some animal products in it. The questions, then, are how much meat do we want, and how will we produce it? The answers depend on how you approach these questions. The most straightforward approach is to argue that the world will continue to want ever more meat. The United Nations' best guess is that by 2050 the world will need to more than double its production of meat, an increase that would be environmentally disastrous.

I Under this scenario, the goal will have to be producing the most meat at the lowest environmental cost. According to Walter Falcon, an agricultural economist, this means no free-range cows and sheep in green fields. "If you're going to keep some livestock systems, I think the ones you'll want to keep are the intensive ones," he says. Of course, this does not take into account animal welfare; intensive farming usually means poor living conditions and the use of growth hormones.

We can choose our friends, the saying goes, but we cannot choose our family of origin. Our parents provide the genetic material and make powerful early role models, but even more influential in determining what kind of adult we will become are our brothers and sisters – our siblings. They occupy a position of unique intimacy in our lives and cast a longer shadow than many recognise or are prepared to acknowledge. By turns enraging and lovable, familiar and mysterious, our brothers and sisters are the human beings who people our first social relationships. And a sibling relationship is the most enduring relationship many of us will ever have – no less emotionally intense than the bonds we form with our spouses and our own children – if only because they started when we were so young.

Surprising new research by Dr Gene Brody found that having older children who do well in school and are well liked by other children leads to parental 'basking' - increasing mothers' self-esteem. It is clear that in turn this is associated with more positive parenting of younger children, who display fewer behavioural problems as a result. Conversely, parents who get a difficult first child may in turn experience a negative spiral of household tension.

International sports star Piri Weepu is a rugby hard man, but at least some of his tough temperament was forged long ago in the family home. His older brother Billy started rugby-tackling little Piri when he was just four - to 'harden him up'. It seemed to work: Piri followed his brother in playing for the under-sevens league before he even started school. There are as many such stories as there are families: in the formative hothouse of the family, siblings interact in complex ways with the powerful forcefields of parents, genetics and personality already at work. It starts with birth order, which determines play roles: the firstborn leader, the middle-child mediator and the rebellious youngest. These are roles that can stick for life.

Clinical psychologist Claire Cartwright says she has clients who believe, and it might not have been true, that they were treated differently from their siblings, that one child in the family was preferred over others. They are quite convinced that the parental judgements on them all those years ago were harsher, and that they were the ones who always got into trouble. Later, in the workplace, such a person might be particularly defensive, and so behave in a way more prone to attracting harsh judgements.

However, the research shows that being Mummy or Daddy's favourite has its own traps. If you were the easy-going, co-operative child at home, you probably excelled at pleasing parents and, later, bosses. But you might, as an adult, find it hard to be assertive with authority figures and lack direction. On the other hand, the sibling who is disruptive and annoying as a child may be able to reinterpret that role later, turning those attention-seeking characteristics into strengths like determination and leadership.

How is it that full siblings, despite sharing DNA, can turn out so differently? One answer is that, in fact, each sibling grows up in a different family, a unique micro-culture. For example, the firstborn is, for a while, an only child, and therefore has a completely different experience of the parents than those born later. The parents themselves are growing up too, weathering hardship or good fortune, so one sibling might experience stability and closeness while another might be raised in the midst of crisis.

Of course, there are many positive aspects of sibling relationships. Annette Henderson, a lecturer in psychology, says firstborn children learn vocabulary more quickly than their siblings because they are not competing to spend one-on-one time with parents. But younger children in turn benefit from unintentional instruction from their bigger brothers and sisters, acquiring entire phrases and an understanding of social concepts such as politeness. Similarly, a Cambridge University study of 140 children found that even when there is a great deal of conflict, siblings will still create a rich world of play and make-believe that extends them developmentally. Love-hate relationships were common among the children, but even those who fought the most had as many positive interactions as the other sibling pairs. It is also true that children compete for parental attention by making themselves different from their brothers and sisters, particularly if they are close in age. A 2003 research paper studied adolescents from 185 families over two years, finding that those who changed, to differentiate themselves from their siblings, increased the amount of warmth they gained from parents, and also developed stronger personalities and a better sense of their own identity.

Another consideration is that many families today are smaller and more intimate than their historical counterparts. This may be advantageous because siblings tend to know each other better and remain lifelong friends compared to children from bigger or more widely spaced families, who perhaps lose touch with one another after leaving home. Then there was a 2010 American study which found that having a sister, whether younger or older, meant that 10- to 14-year-olds were less likely to feel lonely, unloved, self-conscious or fearful. The same study found that having a loving sibling of either gender promoted good deeds, such as helping a neighbour around the house or helping other children at school.