"My parents were very strict with me. I was very mature and I took care of my four siblings - four boys," says Mary [not her real name], a nurse. Growing up as a firstborn child, she felt bossy, confident and a motivated perfectionist. The now 50-year-old divorcee was the first daughter and first child raised under strict rules by her parents.

When she got married, Mary was 28 and said she was looking for someone responsible, who would take care of her and provide for her financially.

"I knew I could take care of myself. But I didn't want to. I was tired of taking care of myself because I did it from a very early age," she notes.

Finding a responsible, mature adult in the husband she chose, Mary married Kevin - a man who happened to be the youngest son of five children.

"Kevin was very responsible but very grumpy, to the point of not even being able to crack jokes," she says. But her level of responsibility began to create problems in their marriage as it turned into control.

And her husband, who wasn't used to being challenged, had problems with her independent nature.

"He was always doubtful about the decisions I made. It always made me feel like I wasn't right. Or that I should question myself," says Mary.

But despite the eroding marriage, she chose to stay. And she stayed. For 21 years.

"Being the leader I was, I tried to make it work. Unknowingly, I was creating a huge rift. I didn't want to be on my own. I was afraid of it," she says of her decision.

According to Prof Catherine Gachutha, Chairperson of the Kenya Counseling and Psychological Association, Mary's marriage had some troubling signs. Her husband came from a well-off family and alcoholic parents, while in contrast all her life Mary has been taking care of little boys growing up being responsible for her siblings.

"I call the oldest the ice cutter in the lake of life. Older children tend to be dominant in a relationship, and for good reason; they know what they want, they get what they want," says Prof Gachutha.

On the other hand, she says, last-born sons are always pampered and used to having things their way.

Study shows that firstborns are well-organized list makers and leaders with lots of practice. They are often held responsible for their siblings. They take life seriously, feeling the pressure to be better and do better.

Prof Gachutha, however, says that all couples should occasionally schedule dates, at least monthly dinners, where they go away from familiar territory and only talk candidly about issues in their relationship or their goals.

"Occasional meetings help individual partners off-load concerns crouching on the mental bandwidth, and help bring closure to any unanswered questions or resolve any misgivings,' she says.

Stephen Kaindi, 56, an only son who is married to a first-born daughter, says at some point he nearly lost his marriage.

"My parents were very lenient towards me. I had everything I wanted when I wanted it. I was treated like a little adult," he says.

"My only child's refusal to compromise on anything, because I never had to during my childhood, enrages my wife who had to step aside for a while," he adds.

According to a survey by the American Association for Marriage and Family Therapy, the hierarchical birth position is implanted deep within children once they are born.

The data has also found that having a relationship in which your birth order position is in a complimentary position to your partner's gives the best chance of long-standing happiness.

It also notes that the highest divorce rates are when an only child marries an only child, as it is usually difficult for them to share the spotlight.

"Early on in life, we all develop a mantra. For an older child, it might be: 'I should be the one in charge'. For the only child it might be: 'I only count in life if I get on with other people.' For the youngest child it might be: 'I may be last, but I am not going to be least'," says Psychologist Dr Ben Mwangi.

''Because birth order is how we learn to relate with others from the start, these experiences get carried forward into how we relate to loved ones in adult relationships," he adds.

Experts believe while firstborns tend to feel they are loved conditionally based on their achievements, they will also be happier if they tell each other they are loved for who they are, not what they achieve.

According to several studies, eldest children are most likely to be unfaithful, possibly in an effort to recreate the intense one-on-one relationship they lost with their parents before their siblings arrived and displaced them.

So within these relationships, it's important for both to help the other feel valued and unique. A 2018 survey carried out in Kenya by research firm Consumer Insight confirms that 24 per cent of those in romantic relationships admitted to cheating on their partners.

Globally, recent statistics by the American Association for Marriage and Family Therapy indicate that 15 per cent of married women and 25 per cent of married men have had extramarital affairs.

"If you often clash over decisions, understand the deep-seated reasons which make you both feel you must be right," says Dama Khonde, an 80-year-old first-born daughter who has been married to a first-born son for six decades.

However, she says this does not mean firstborns always have an easy time in a relationship. "As long as love flourishes between two firstborns, all might be hunky-dory," says Ms Khonde.