With deadlines, is it best to set a tight one, or one further into the future, to allow for more time to complete the task? Or perhaps no deadline at all works best? That's the question posed by a new study, and the results might surprise you.

Participants for the study, led by researchers from the University of Otago in New Zealand, were selected at random from the New Zealand electoral roll. Each person received a letter asking them to complete an online survey on charitable giving, which would take around five minutes to complete. In return, they would get a small monetary reward that they could donate to either World Vision or the Salvation Army.

Three different mailings were sent out, covering 1,092 people in each case: one giving a deadline of 10 days after the letters were sent (so leaving 7-9 days after delivery), one adding an extra three weeks on to that, and one specifying no deadline at all.

You might want to have a guess at which group returned most responses before we give away the results. In the meantime, a word from the research team.

"Appropriately selected deadlines can signal urgency and importance of the task, which increases the chance of completion as people are likely to get things done early on," says economist Maroš Servátka, from the Macquarie Business School in Australia.

"This is particularly important in situations that do not feature natural reminders because once people postpone the task, they might forget about it."

Curiously, the response rate was highest – with 8.32 percent of people completing the survey – when no deadline was given. It seems that in this scenario at least, adding a sense of urgency to the task wasn't going to result in any more responses coming back.

With a deadline of a week, 6.59 percent of people completed the survey (including three surveys that came in after the deadline). Of those given a deadline of a month, only 5.53 percent of participants entered a response online.

The month deadline also saw the slowest responses, with the most speedy survey completions coming from the people with the one week deadline or no deadline at all – many of whom filled out the survey in the first couple of days of learning about it.

It might seem logical that allowing more time for a survey to be completed would mean more responses coming back – but it's interesting that trying to convey urgency and importance with a deadline can be counterproductive, especially if the deadline is longer than a few days.

"Importantly for our main results, the No Deadline treatment features a spike of responses at the beginning and a long tail of later responses, while the One Month treatment features almost no responses between day 14 and day 27, but a little spike on days 27 to 30," write the researchers in their published paper.

"The spike could be evidence of a small number of people either being attentive or possibly being sophisticated and setting a reminder for themselves."

The study backs up previous work suggesting that having no deadline gives forgetful and inattentive people more opportunity to remember something that they meant to do – especially if there's a physical letter on the table or the fridge reminding them about it.

It's worth bearing in mind that this is only one type of task: Deadlines of various lengths might well produce different results if the participants in the study were writing essays, or filling out tax returns, or getting around to painting the bathroom.

However, the study is certainly interesting in how it shows the benefit of no deadline at all in these scenarios. If charities are keen to get as much help and support from the public as possible, then they might want to avoid setting longer deadlines of a month, and perhaps avoid setting any deadline at all.

"We interpret this as evidence that specifying a longer deadline as opposed to a short deadline or no deadline at all, removes the urgency to act, which is often perceived by people, when asked to help," conclude the researchers.

"People therefore put off undertaking the task, and since they are inattentive or forget, postponing results in lower response rates."

The research has been published in Economic Inquiry.