McGEE, Justice.
This is a product defect case involving an alleged defect in the design of a 16-foot bass boat. The plaintiffs were the widow and adult children of Samuel Bailey, who was killed in a boating accident in May of 1973. They sued under the wrongful death statute, alleging that Samuel Bailey's death occurred because the boat he was operating was defectively designed. The boat had struck a partially submerged tree stump, and Bailey was thrown into the water. With its motor still running, the boat turned sharply and circled back toward the stump. Bailey was killed by the propeller, but it is unclear whether he was struck when first thrown out or after the boat circled back toward him.

EVIDENCE OF DESIGN DEFECT

The alleged design defects are causally related to Bailey's being thrown from the boat and struck by the propeller and not to the boat's hitting the stump. Nevertheless, the same rules of strict liability govern cases in which the defect caused the initial accident and cases in which the defect caused the injuries. Turner v. General Motors Corp., 584 S.W.2d 844, 848 (Tex. 1979).

In Turner v. General Motors Corp., this court discussed the strict liability standard of “defectiveness” as applied in design defect cases. Whether a product *746 was defectively designed requires a balancing by the jury of its utility against the likelihood of and gravity of injury from its use. The jury may consider many factors before deciding whether a product's usefulness or desirability are outweighed by its risks. Their finding on defectiveness may be influenced by evidence of a safer design that would have prevented the injury.[FN2] Turner v. General Motors Corp., supra at 849. See Keeton, Product Liability and the Meaning of Defect, 5 St. Mary's L.J. 30, 38 (1973); Wade, Strict Tort Liability of Manufacturers, 19 Sw.L.J. 5, 17 (1965). Because defectiveness of the product in question is determined in relation to safer alternatives, the fact that its risks could be diminished easily or cheaply may greatly influence the outcome of the case.

Whether a product was defectively designed must be judged against the technological context existing at the time of its manufacture. Thus, when the plaintiff alleges that a product was defectively designed because it lacked a specific feature, attention may become focused on the feasibility of that feature the capacity to provide the feature without greatly increasing the product's cost or impairing usefulness. This feasibility is a relative, not an absolute, concept; the more scientifically and economically feasible the alternative was, the more likely that a jury may find that the product was defectively designed. A plaintiff may advance the argument that a safer alternative was feasible with evidence that it was in actual use or was available at the time of manufacture. Feasibility may also be shown with evidence of the scientific and economic capacity to develop the safer alternative. Thus, evidence of the actual use of, or capacity to use, safer alternatives is relevant insofar as it depicts the available scientific knowledge and the practicalities of applying that knowledge to a product's design. This method of presenting evidence of defective design is not new to the Texas law of product liability. See, e. g., Rourke v. Garza, 530 S.W.2d 794 (Tex. 1975); Henderson v. Ford Motor Co., 519 S.W.2d 87 (Tex. 1974); Williams v. General Motors Corp., 501 S.W.2d 930 (Tex.Civ.App. Houston (1st Dist.) 1973, writ ref'd n.r.e.); Hartzell Propeller Co. v. Alexander, 485 S.W.2d 943 (Tex.Civ.App. Waco 1972, writ ref'd n.r.e.); Pizza Inn, Inc. v. Tiffany, 454 S.W.2d 420 (Tex.Civ.App. Waco 1970, no writ). #p#分页标题#e#

As part of their case-in-chief, the Baileys produced evidence of the scientific and economic feasibility of a design that would have caused the boat's motor to automatically shut off when Bailey fell out. According to the Baileys, the boat's design should have incorporated an automatic cut-off system or the boat should have been equipped with a safety device known as a “kill switch.”

The deposition of J. C. Nessmith, president of Boatland, was read, in which he stated that there were presently several types of “kill switches” available, and that they were now installed by Boatland when it assembled and sold bass boats.

The deposition of Bill Smith, who was a passenger in the boat with Bailey at the time of the accident, was also read. Smith had not heard of automatic kill switches before the accident, but afterwards he got one for his own boat.

Boatland elicited evidence to rebut the Baileys' evidence of the feasibility of equipping boats with kill switches or similar devices in March of 1973, when the boat was assembled and sold. The Baileys had been granted a running objection to all evidence of this nature. In response to the Baileys' evidence that kill switches were presently used by Boatland, Nessmith testified that he did not know of kill switches until the spring of 1973, and first began to sell them a year later.

In response to the Baileys' evidence that the “Quick Kill” was readily available at the time of trial, Horton stated on cross-examination that until he obtained the patent for his “Quick Kill” in 1974 he kept the idea to himself. Before he began to manufacture them, he investigated the market for competitive devices and found none. The only applications of the automatic engine shut-off concept in use at the time were homemade, such as on racing boats. He first became aware of competitive devices in August of 1974.

Boatland introduced other evidence to show that kill switches were not available when Bailey's boat was sold. The deposition of Jimmy Wood, a game warden, was read in which he stated that he first became aware of kill switches in 1975. He testified that he had a “Quick Kill” on his boat since 1976, and he thought it was the only kill switch made. Willis Hudson, who manufactured the boat operated by Bailey, testified that he first became aware of kill switches in 1974 or 1975 and to his knowledge no such thing was available before then. Ralph Cornelius, the vice-president of a marine appliance dealership, testified that kill switches were not available in 1973. The first kill switch he saw to be sold was in 1974, although homemade “crash throttles” or foot buttons had long been in use.

Apart from evidence of the feasibility of an automatic motor cut-off design, evidence was introduced pertaining to whether such a design would have prevented Bailey's injuries. After considering the feasibility and effectiveness of an alternative design and other factors such as the utility and risk, the jury found that the boat was not defective. The trial court rendered judgment for Boatland. The Baileys complained on appeal that the trial court erred in admitting Boatland's evidence that kill switches were unavailable when Bailey's boat was assembled and sold. The court of civil appeals agreed, holding that the evidence was material only to the care exercised by Boatland and thus irrelevant in a strict liability case. #p#分页标题#e#

In its appeal to this court, Boatland contends that the court of civil appeals misconstrued the nature and purpose of its evidence. According to Boatland, when the Baileys introduced evidence that kill *748 switches were a feasible safety alternative, Boatland was entitled to introduce evidence that kill switches were not yet available when Bailey's boat was sold and thus were not a feasible design alternative at that time.

The primary dispute concerning the feasibility of an alternative design for Bailey's boat was the “state of the art” when the boat was sold. The admissibility and effect of “state of the art” evidence has been a subject of controversy in both negligence and strict product liability cases. In negligence cases, the reasonableness of the defendant's conduct in placing the product on the market is in issue. Evidence of industry customs at the time of manufacture may be offered by either party for the purpose of comparing the defendant's conduct with industry customs. An offer of evidence of the defendant's compliance with custom to rebut evidence of its negligence has been described as the “state of the art defense.” See generally 2 L. Frumer & M. Friedman, Products Liability s 16A(4)(i) (1980). In this connection, it is argued that the state of the art is equivalent to industry custom and is relevant only to the issue of the defendant's negligence and irrelevant to a strict liability theory of recovery.

In our view, “custom” is distinguishable from “state of the art.” The state of the art with respect to a particular product refers to the technological environment at the time of its manufacture. This technological environment includes the scientific knowledge, economic feasibility, and the practicalities of implementation when the product was manufactured. Evidence of this nature is important in determining whether a safer design was feasible. The limitations imposed by the state of the art at the time of manufacture may affect the feasibility of a safer design. Evidence of the state of the art in design defect cases has been discussed and held admissible in other jurisdictions. See, e. g., Raney v. Honeywell, Inc., 540 F.2d 932 (8th Cir. 1976); Caterpillar Tractor Co. v. Beck, 593 P.2d 871 (Alaska 1979); Barker v. Lull Engineering Co., 20 Cal.3d 413, 573 P.2d 443, 143 Cal.Rptr. 225 (1978); Kerns v. Engelke, 76 Ill.2d 154, 28 Ill.Dec. 500, 390 N.E.2d 859 (1979); Cepeda v. Cumberland Engineering Co., Inc., 76 N.J. 152, 386 A.2d 816 (1978). See generally J. Sales & J. Perdue, The Law of Strict Tort Liability in Texas 41 (1977). Note, The State of the Art Defense in Strict Products Liability, 57 Marq.L.Rev. 491 (1974). Note, Product Liability Reform Proposals: The State of the Art Defense, 43 Albany L.Rev. 944, 944-45 (1979). In this case, the evidence advanced by both parties was relevant to the feasibility of designing bass boats to shut off automatically if the operator fell out, or more specifically, the feasibility of equipping bass boats with safety switches. #p#分页标题#e#

The Baileys offered state of the art evidence to establish the feasibility of a more safely designed boat: They established that when Bailey's boat was sold in 1973, the general concept of a boat designed so that its motor would automatically cut off had been applied for years on racing boats. One kill switch, the “Quick Kill,” was invented at that time and required no mechanical breakthrough. The Baileys were also allowed to show that other kill switches were presently in use and that the defendant itself presently installed them.

Logically, the plaintiff's strongest evidence of feasibility of an alternative design is its actual use by the defendant or others at the time of manufacture. Even if a safer alternative was not being used, evidence that it was available, known about, or capable of being developed is relevant in determining its feasibility. In contrast, the defendant's strongest rebuttal evidence is that a particular design alternative was impossible due to the state of the art. Yet the defendant's ability to rebut the plaintiff's evidence is not limited to showing that a particular alternative was impossible; it is entitled to rebut the plaintiff's evidence of feasibility with evidence of limitations on feasibility. A suggested alternative may be invented or discovered but not be feasible for use because of the time necessary for its application and implementation. Also, a suggested alternative may be available, but *749 impractical for reasons such as greatly increased cost or impairment of the product's usefulness. When the plaintiff has introduced evidence that a safer alternative was feasible because it was used, the defendant may then introduce contradictory evidence that it was not used.

Thus in response to the Baileys' evidence of kill switch use in 1978, the time of trial, Boatland was properly allowed to show that they were not used when the boat was sold in 1973. To rebut proof that safety switches were possible and feasible when Bailey's boat was sold because the underlying concept was known and the “Quick Kill,” a simple, inexpensive device had been invented, Boatland was properly allowed to show that neither the “Quick Kill” nor any other kill switch was available at that time.

It could reasonably be inferred from this evidence that although the underlying concept of automatic motor cut-off devices was not new, kill switches were not as feasible an alternative as the Baileys' evidence implied. Boatland did not offer evidence of technological impossibility or absolute nonfeasibility; its evidence was offered to show limited availability when the boat was sold. Once the jury was informed of the state of the art, it was able to consider the extent to which it was feasible to incorporate an automatic cut-off device or similar design characteristic into Bailey's boat. The feasibility and effectiveness of a safer design and other factors such as utility and risk, were properly considered by the jury before it ultimately concluded that the boat sold to Bailey was not defectively designed.[FN3] #p#分页标题#e#

In cases involving strict liability for defective design, liability is determined by the product's defective condition; there is no need to prove that the defendant's conduct was negligent. Considerations such as the utility and risk of the product in question and the feasibility of safer alternatives are presented according to the facts as they are proved to be, not according to the defendant's perceptions. Thus, even though the defendant has exercised due care his product may be found defective. When the Baileys introduced evidence of the use of kill switches, Boatland was entitled to introduce rebuttal evidence of nonuse at the time of manufacture due to limitations imposed by the state of the art. Evidence offered under these circumstances is offered to rebut plaintiff's evidence that a safer alternative was feasible and is relevant to defectiveness. It was not offered to show that a custom existed or to infer the defendant's compliance therewith. We would be presented with a different question if the state of the art in 1973 with respect to kill switches had not been disputed and Boatland had attempted to avoid liability by offering proof that Bailey's boat complied with industry custom.

For the reasons stated above the judgment of the court of civil appeals is reversed. The judgment rendered by the trial court, that the Baileys take nothing against Boatland, is affirmed.