A new psychology study investigating the best way to say 'thank you' has now offered a few preliminary tips for those looking to strengthen their romantic relationships.

In both diary entries and conversation studies, psychologists found the best way to express gratitude is to elaborate on how responsive your loved one was to your needs.

For instance, saying something like, "I wouldn't have made it to the meeting on time if you didn't drop me off at the office today", tended to produce the most positive response from a partner in the lab and in daily life.

No matter the tone or affect of the speaker, the more that a 'thank you' conveyed how much someone was valued and included in their partner's goals, "the more positively the benefactor perceived the gratitude and felt toward their relationship".

Meanwhile, highlighting the extent of a partner's sacrifice – for instance, "I know it was a hassle for you to drop me off at my office during rush hour" – was not as effective as a form of praise.

"These findings are consistent with the notion that romantic relationships are communal in nature and that giving and receiving of benefits between partners are based on responsiveness to each other's needs," the authors write.

"That is, benefactors are satisfied to learn that what they did met their partner's needs precisely because their behaviour was motivated by a concern for their partner's welfare."

Gratitude has long been celebrated as a virtue, and in the past two decades or so, accumulating evidence suggests this positive emotion could play an important role in human society.

This is called the find-remind-bind theory, and it essentially posits that expressions of gratitude are evolutionarily advantageous because they reinforce bonds between humans.

Even research on primates indicates gratitude is more than a simple cultural construct; it appears to be present in our very evolutionary history. Studies show that a chimp is more likely to share its food with another who groomed them or helped them out in the past.

We humans, it turns out, are not so different. Recent correlational evidence suggests gratitude is important in personal and relational growth, possibly more so than other positive emotions, which generally have, well, positive effects.

But not all expressions of gratitude are constructive. Grand gestures can make another person feel indebted or guilty. In some situations saying 'thank you' can even be seen as embarrassing or insulting.

Learning how to do it right is something we could all benefit from, but few studies have actually examined how to express gratitude in a way that improves or strengthens an established relationship.

One of these studies, which was published in 2016, found that when people were asked to praise their romantic partner, they reported higher wellbeing.

The current research helps to flesh out this finding. Among 111 participants who had been in a relationship for an average of 4 and a half years, researchers compared two different expressions of gratitude. One conveyed how responsive a partner was to their needs, and another conveyed how much the act of kindness cost the partner.

The diverse group of young couples, who were on average about 27 years of age, first completed an online questionnaire and were then brought into the lab to hold three filmed conversations on different topics, which they later reported felt natural.

In the final conversation, participants were given one minute to talk about something nice or kind the other had done for them, which made them feel particularly thankful or appreciative.

Videos were then assessed by a team of five research assistants who had not been informed about the purpose of the study. Seeing only the speaker, these assistants ranked the speeches based on how much their gratitude highlighted the cost to their partner and how much it highlighted their partner's responsiveness.

After completing another questionnaire about how their partner's gratitude made them feel, participants were sent home and instructed to complete a short survey each evening for two weeks, reflecting on the sacrifices or compromises they made that day and on their own perceptions of the gratitude they received.

In the end, the 'diary' portion of the study and the conversation studies showed similar results.

Acknowledging the degree to which a partner responds to your needs - rather than the degree to which it was costly - appeared to have a more positive effect on the relationship as a whole.

"Of course, our results do not suggest that acknowledgment of the costs has no value to the benefactor in a communal relationship; it may still be a better alternative to not expressing gratitude at all," the authors explain.

"Yet, once feelings of gratitude have been conveyed, it may be the message of how much they have made an impact in the partner's wellbeing that is more satisfying to the benefactor."

But it could also have something to do with this particular type of relationship. The partners included in this study had been together for at least a year, and sacrifices at this stage of a relationship may be assumed.

"Perhaps when relationship norms and expectations have not been established yet and concerns about being taken advantage of are more salient, there may be some benefits of receiving explicit acknowledgment of one's efforts," the authors muse.

Now, it's important to note that studying conversations among people is very difficult, and chats carried out in the lab may not be typical of what would occur naturally. Larger studies will need to replicate the results, of course, especially if we want to see whether this applies to other types of relationships, like those with our friends or family.

That said, if gratitude really does serve as the backbone of human society, as it's sometimes said, then it makes a lot of sense to study more.

The study was published in the Journal of Social and Personal Relationships.