1. I favour an eclectic approach in psychology and I see these as the foundation of good mental health:
- no physical or genetic anomalies associated with mental problems (please note, physical defects on their own do not necessarily lead to poor mental health, however, they increase the risks);
- good enough mother: This concept refers to a caregiver (not necessarily a biological mother) who provides a child with adequate care and emotional support, particular styles of care and communication do not matter as long as the caregiver-child relationship is comfortable for both parties and is based on their needs (must not be codependent, though);
- adaptive coping mechanisms: This refers to behavioural and cognitive strategies that help to process negative emotions, adaptive coping leads to inner peace, i.e. low stress, sufficient self-esteem, etc.;
- meaningful relationships with other people: Humans are social beings and do best when they have positive emotional connections with other people, isolation from society is very harmful;
- balance between work and pleasure: People need rest, lack of rest and relaxation is detrimental for mental and physical health.
In addition to these, a life of stability (no sudden and dramatic changes) and structure can also be beneficial. I would also argue that well-defined social roles help the majority of people to stay mentally healthy and live happier (no angst of choice). However, overrigid structures and roles can be detrimental, especially for people who have rebellious or novelty-seeking personalities.
2. There are several different ways to define childhood. My working definition is based on social roles: A child stops being a child when they assume adult social roles. For this question, this means that children become adults at the age of 12. The period between 12 and early 20s will be called Young Adulthood. Adolescence will not exist in the described scenario.
Part 1. Childhood (0-12)
In our modern world children are often seen as pure, innocent, naive, and in need of a 'safe' environment. However, these are very new and novel concepts that did not exist before modernity. Throughout history, children were treated as small adults with their own set of well-defined social roles and responsibilities. One may even argue that the current obsession with safety and practices of sheltering children from everything (and resulting lack of agency and autonomy) have a much greater traumatising potential than previous historical attitudes.
For your setting, I would suggest going back to treating children as mini-adults:
- minimal sheltering;
- no hiding of ugly truths;
- early involvement in society as full members with defined and respected responsibilities (can be looking after younger children, housework, light public work, and alike).
This approach will minimise the shock related to the transition from childhood to adulthood. It will also help to develop and support decision autonomy and personal agency. Both are very important for mental health.
An additional benefit is the full (or almost full) disappearance of the adolescence crisis, which happens in our societies due to the clash of personal identities, responsibilities, available roles, and lack of agency. To put it simpler, adolescents rebel because they believe that they are already adults but they are still treated as children and are not allowed to decide on their own. This type of rebellion is very rare in poor families and regions where children take on adult roles early.
Your society can also adopt initiation rites. They will provide structure and clarity, facilitate coping with new social roles and requirements. Initiation may also work as a pre-emptive coping mechanism when it comes to war trauma: An initiation ritual may include the symbolic (or real) killing of an enemy which will prepare for actual battles. I would suggest looking into initiation rites in different societies and associated rituals.
Everything else is just common sense:
- limit corporal punishments of children (it is hard for me to say what is a good limit as I come from a culture where domestic corporal punishments are a norm [I never punished my children physically, but when I was a child, I and most people I knew at that time received such punishments]; I am also unaware of any research that would conclusively state that any type of corporal punishment is 100% traumatic for children; I do agree that heavy beatings should be strictly forbidden and punished);
- public schools (children need a place to learn and to socialise);
- basic siege training (may include basic military training, evacuation drills, basic medical training, etc.);
- playgrounds for younger children (unless your society is in dire need of workforce and children have absolutely no time for playing; please note, that even in the middle ages children had some free time to play);
- provide all basic necessities (food, water, and shelter);
- interest groups, clubs, etc. (these are very helpful for children to find like-minded individuals and to figure out what they are interested in, these groups can also become a place to look for and nurture talents).
Part 2. Young Adulthood (12-25)
Children become young adults when they assume adult social roles and responsibilities.
This part will focus on war.
For children and young adults born and raised during the siege, everything associated with it will be the norm: Bombing, poisonous gas, people dying, wounds, etc. None of these will be traumatising events as these are facts of everyday life. I also do not think that being underground will cause any psychological problems (except for the first generation that went underground).
Malnutrition and undernutrition may become a problem. Studies suggest that they lead to a wide variety of negative effects which may persist through generations (the last link is to Google Scholar search; for those interested in exploring this in-depth). These are not psychological traumas or mental health problems per se, but this is something that should be considered.
As for psychological problems, I think that the most important will be:
- inability to leave the city;
- fatigue due to neverending war;
- trauma associated with combat;
- death of close friends and relatives.
Inability to leave the city is the least traumatic. The core of the problem lays within a lack of decision power. People have to stay inside the city whether they want it or not. I would expect the majority of people to cope with this on their own. However, there will be some adventurous individuals that will struggle. The easiest solution would be to employ them as scouts: Send them to spy on enemy's movement, look for resources, make maps, etc. Let them explore and enjoy freedom.
Mental and emotional fatigue and apathy when it comes to war will be the norm. It is not possible to maintain high morale and due diligence when war is going on for decades. This may even cause social friction between those who fought on the front lines and those who did not.
I would suggest a combination of propaganda with troop rotation. Change propagandist messages often, create stories of heroes, invite soldiers to schools and public gathering places. If an aggressive militarized culture works for you, it might be the best option. Every young adult will see participation in combat as their honour. Those who are traumatised by leaving their homes will be a rarity.
Troop rotation is very important for morale and diligence. Soldiers need rest away from a battlefield. They need at least some time where they can sleep, eat, and joke without any possibility of interruption. Make a rotation schedule and keep it.
Psychological trauma associated with combat will most likely manifest as PTSD, survivor's guilt, and depression. All of these need therapy1.
I checked research on PTSD to see if ages 12-25 are at higher risk of developing PTSD symptoms. Unfortunately, the results are inconclusive. One of the papers mentioned that PTSD in adolescents (15-21) can be related to identity issues that will be non-existent in your setting.
The prevalence of PTSD among Iraq and Afghanistan military veterans is estimated to be between 13 and 20% with some estimates going as high as 30%. The paper suggests that conventional combat is less likely to cause trauma.
In our world, psychotherapy in combination with medications results in full remission for 30 to 50% of all PTSD cases and noticeable improvement in the majority of cases.
In your world, psychotherapy is the only solution. Success rates will be lower. (There is also a possibility that in some cases PTSD symptoms can resolve after repeated exposure to traumatic events.) It does not mean that all veterans affected by PTSD will be permanently disabled and will become unproductive members of society. Not all cases are extreme and many symptoms are manageable. Your society also should be more tolerant and accepting of war veterans and their problems, which should alleviate the situation as well.
Depression and survivor's guilt are often chronic conditions. They can be managed with psychotherapy.
I would suggest psychological consulting after each deployment. Many traumas can be resolved if found early. Soldiers who show even minor symptoms of psychological trauma or distress should be placed into therapy. Access to alcohol (and drugs if any exist in your setting) should be limited since patients with PTSD and depressions often form substance-use disorders.
The death of close friends and family is always traumatic. However, I believe that in your setting people will be able to cope with it better since death is so common. Therapy should be needed only in exceptional cases. Funerals help the living to sort out their emotions and say their final goodbyes to the deceased. So, make sure your society has a funeral culture. Glorifying war sacrifice can also help as people will feel grief but also pride and honour of being related to war heroes. The Russian attitude toward the Great Patriotic War (1941-1945) can be of interest here and provide some insights.
All age groups should have access to recreation. If people are unable to relax, their survival will be compromised. No matter how desperate the struggle is people need time when they can forget about everyday matters. The type of recreation depends on your world's specifics. I would say that social activities where people interact with each other and form meaningful relationships are much healthier than many other options. Perhaps, you could do some public discussion events, dance parties, social parties, sports competitions, etc.
In addition, healthy drinking culture and relative freedom of sex should help. Drinking with friends may have a therapeutic effect if it allows opening one's heart, relaxation, and experiencing an emotional connection with other people. Drinking is also helpful when it comes to bonding: It is easier to make friends when everybody is slightly intoxicated. Healthy drinking culture may also decrease the prevalence of alcoholism. The specifics of such a drinking culture depend on your personal preferences.
The role of sex is rarely mentioned. However, sex does help with bonding and mental relaxation. Whether you want to establish brothels2 or not and how they work is entirely up to you. I would only warn against adopting negative attitudes toward sex and forbidding sex outside of marriage. Let young people enjoy themselves as much as they can while they are still alive. It will also help with birth rates :)
Humans are very resilient as a species and can adapt to a wide variety of circumstances and environments. In a setting where war is always at your doorstep those who cannot cope will die very soon. It might be a cruel thought, but it is a good thing for your society as it will become stronger and fitter.
However, it will only work if people have hope and they see their lives as meaningful. Only under these conditions do people have the will to live. Survival is not the same as living a full life. It is not enough to survive. Without knowing why do you live, survival is meaningless. Without hope, life becomes a nightmare.
This answer is already extremely long, so I will not elaborate further. I would suggest reading about existentialism to get some idea of how meaning can be approached.
IMO, in your scenario, it would make sense to make raising children a collective responsibility. Considering high casualty rates, nuclear families (only parents and children) might not be possible. Extended families (several generations living together) may also be insufficient if grandparents do not live long enough. You might consider developing a public system that would supplement and replace when necessary parental care. Also, orphanages have a bad reputation but they might be a necessity in your setting.
1 There are many different types of psychotherapy. What works for some people does not work for others. If possible, have therapists that practise different approaches. Also, support groups (without a therapist) and social workers can help with cases that need prolonged intervention and monitoring. A lot of people benefit simply from sharing their thoughts and experiences with people who can relate to them. Social workers help with social adaptation and mundane problems that someone affected by mental trauma can experience after returning from the front lines (this makes a person feel cared for, not isolated, not so lonely and non-understood, i.e good for mental health). Social workers will also be of invaluable help when it comes to orphans.
2 Prostitution is a highly politicised topic these days, so any research should be viewed very sceptically. Some studies claim that legal prostitution reduces the prevalence of STDs and rape. Other studies suggest the opposite. The morality of this issue is another widely discussed aspect.
It is my personal opinion that sex work can be performed safely and in a dignified manner if sex workers are given enough agency, coercion is strictly forbidden and heavily persecuted, and there is a well-thought-out and fully enforced labour code to protect the rights of sex workers. It is also important to have social attitudes that respect and do not denigrate sex workers.