If I were a woman, I think I’d rather have twins than two children one at a time. The costs of having children can be divided into two main categories: child-bearing costs and child-rearing costs. With two children carried one at a time, the child-bearing costs are incurred twice. With twins, the childbirth costs are incurred only once. Thus, twins allow one to spread child-bearing costs. Think of it: just one body-morphing pregnancy, just one set of visits to the obstetrician, just one set of maternity clothing…
But what about the child-rearing costs? Raising two infants is assuredly more difficult than raising just one. Yet if it’s less than twice as difficult, then you’re still reducing your child-rearing costs by having kids in tandem instead of in sequence. In reality, it’s probably more than twice as costly, since some costs can only be shared over time, not simultaneously: twins require two cribs, two sets of clothes (no hand-me-downs!), and so on. These factors might well increase the overall child-rearing cost of twins relative to infants-in-sequence. Nonetheless, I surmise that the savings from spreading the child-bearing costs would more than offset the increment in child-rearing costs.
I’ve been told that twins have a nasty tendency to wake each other up, which means that parents of twins get even less sleep. I think this buttresses my point. Say a typical infant has a 50% chance of waking during any given hour of the night, for an 8-hour night. On average, then, the parents get wakened 4 times per night. With twins, there is a 75% chance of one or both infants waking in a given hour, so on average parents get wakened 6 times per night. Now, that’s clearly bad. But which would you rather have: 12 months of being wakened 4 times per night, or 6 months of being wakened 6 times per night? The former involves 25% more wakenings than the latter. Now, I grant that on a given night, there may be increasing marginal disutility of wakenings; the 5th and 6th wakenings are probably much more wearisome than the first 4. But are they so much worse that you’d willingly double the number of nights that you get wakened at all? I figure that once I've been wakened 4 times, my night is shot anyhow, so I’d rather concentrate my days of sleeplessness into a shorter period of time. (Update added later: I realize that I’m ignoring that wakened twins may require two parents instead of one to get out of bed. But I still think I’d rather concentrate my misery in a shorter period.)
I say all of this, of course, as a single man without one iota of experience in parenting. I therefore invite any qualified parents to refute my claim. But remember, the only parents qualified to make an empirical comparison are those who have had four children, with one set of twins and two single births!